CANOPY ARCHITECTURE

Canopy architecture

" The overall profile of the tree top is known as canopy architecture".

Vegetation canopy structure describes the three-dimensional arrangement of leaves and includes such canopy attributes as canopy architecture, leaf angle distribution, ground cover fraction, leaf morphology, vegetation spatial heterogeneity, and shadows.

Canopy architectures in plants:-

Canopy architecture is the overall profile of tree the  tops. The trees gets grouped together leading to the formation of canopy and reduces the amount of light beneath them.

Factors controlling the canopy architecture:-

1. Pattern of growth in plant axis.

2. Arrangements and distribution of branches on main shoot.

3. Angle of branches on shoot.

4. Direction of branches.

5. Size, arrangement, distribution and shapes of leaves.

6. Genetic, environmental and nutritional factors.

Heliophytes or shade-introlerant plants- those plants which grows in full sunlight. eg. Sunflower, poplars, pines etc.

Sciophytes or shade-tolerant plants- certain plants which survive only under shade. eg. Fir, spruce etc.

However some plants can be facultative sciophytes as well as facultative heliophytes.

fig : Heliophytes and Sciophytes

The forest is made up of some individual tall plants which protrudes above normal level of canopy and are called as emergents.

The process of stratification i.e. arrangements of different plants or animals in different layer of substratum takes place in canopy.

Eg. Stratification in a tropical rain forest.

Based upon branching a tree, it may classified as:-

1. Culm- The stem is unbranched showing distinct nodes and internodes imparting a jointed apperance. Eg. Bambusa arundinacea (Bamboo).

2. Columnar (caudex)- In such case trunk is unbranched and bears a crown of leaves at its apex. Eg. Fan plam, areca catechu (supari) , Cocos nucifera (coconut) etc.

3. Excurrent- In such case lateral branches do not complete with the growth of main stem. Branches appear in acropetal succession. Thus, tree gives cone-like apperance. Eg. Pinus, Eucalyptus etc.

4. Decurrent or Deliquescent- When some of the lateral branches grows vigorously, the trunk Disappears after some distance due to the suppression of growth of terminal bud, the form called Decurrent. Eg. Magnifera indica (mango), Dalbergia(Shisham), Ficus benghalensis etc.

SHAPE OF CANOPY ARCHITECTURE

a. Round canopy- Trees which shows Decurrent or Deliquescent branching shows round canopy. Branch appears in alternate pattern. Branches present on upper side grows towards upper and outer sides. However, lower branches spread  out. Minimum overlapping of leaves in such cases. Eg. Magnifera indica , lagerstromia, Dalbergia, Ficus etc.

b. Conical canopy- In such case lateral branches do not complete with the growth of main stem. Branches appears acropetally (Monopodial/Racemose).

Branches appear either horizontally (eg. Araucaria) or ascent at a particular angle (eg. Thuja). Shape of tree is conical.

Eg. Pinus, Eucalyptus etc.

c. Oblong or cylindrical canopy- Branching in these trees can be Racemose (Gnetum) or cymose type (peltophorum). Main trunk remains upright. Lateral branches are short and grows towards upper side providing cylindrical shape to trees. 

Eg. Artocorpus (jack fruit).

d. Flat topped canopy- Growth of apical bud on main trunk stops after attaining a certain height. It bends on upper side, a twig appears which further give rise to Obliquely rising branches. Thus, tree imparts umbrella shape. Eg. Cassia, Terminalia (imli), Thespesia actifolia, Enterolobium saman (rain tree) etc.

e. Weeping tree- Due to weak branching or due to drooping branching pattern, trees impact weeping appearance.

Stem is Monopodial (Polyalthia) or stem is sympodial (Zizyphus jujuba).

Eg. Salix (weeping willow), Acacia arabica (Kikar).

f. Pagoda trees- Trees bear spirally arranged compacts, simple or palmate type of leaves. The branches on trunk arises in wholed fashion. The pagoda shape is very distinct when the tree is young. As the whorls ascent from the base to tip, length internode shortens and gives a typical pagoda shape to trees. Eg. Salmalia, Achras sapota (Chiku), Terminalis etc.

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INTERNAL STRUCTURE OF YOUNG ROOT

INTERNAL STRUCTURE OF DICOTYLEDONOUS ROOT

Internal structure of a typical dicotyledon root shows following features.

Epiblema/Epidermis- it is unseriate (single layered) outermost layer. Cuticle and stomata are absent.

Many of the epidermal cells protrude in the form of unicellular root hairs means unicellular root hairs are formed due to elongation of cells of epiblema. Epiblema is also known as Rhizodermis  or Piliferous layer. Root hairs are present in maturation  zone of root. Root hairs ate short lived.

Note : Cells of epiblema which develop root hairs are called trichoblast cells.

Hypodermis is absent in the roots.

Cortex- it is made up of the walled parenchymatous cells with intercellular space.

Chloroplast is absent, so they are non photosynthetic but chloroplast are present in roots of Tinospora, Trapa and Taeniophyllum,so  they are photosynthetic (Assimilatory roots).

Endodermis- the innermost layer of the cortex is called endodermis. It comprises a single layer of barrel shaped cells without any intercellular soace. This layer of barrel shaped cells is situated between the pericycle and cortex. Casparian strips/bands are present on radial and tangential walls of endodermis.

These strips  are made up of ligno suberin means lignin & suberin (mainly suberin).suberin is water impermiable waxy material. Casparian strips were discovered by caspari.

The cells of endodermis which are situated infront of protoxylem cells are devoid of casparian strips. These are called passage cells/path cells.

Passage cells provide path to absorbed water from cortex to pericycle.

Intercellular spaces are absent between the cells of endodermis of root.

Endodermis acts as a water tight jacket (Dam) which prevents leakage of water from stele.

Pericycle:- it is a single or few layers of thick walled parenchymatous cells (it is composed of prodenchyma).

Lateral roots usually originate from the part of pericycle which is lying opposite to protoxylem. Thus lateral roots are endogenous in origin.

Note:

1. The branches of stem (vegetative branches) are exogenous in origin because they originate from extrastelar region.

2. Adventitious roots are endogenous in origin because they originate from stelar region.

Vascular Bundles- vascular Bundles are radial and exarch. Xylem and phloem are separate and equal in number. The number of xylem patches and phloem patches are usually two to four but they may be two to six (diarch to hexarch). Tetrarch condition is found in gram and sunflower.

Parenchyma which is found between the xylem and phloem is called comjuctive tissue.

In dicot root xylem vessels appear angular (polygonal/hexagonal) in T.S.

Pith- in dicot root pith is small (less developed) or inconspicuous or absent.

INTERNAL STRUCTURE OF MONOCOTYLEDONS ROOT

The internal structure of a typical monocotyledons root is similar to Dicotyledons root except some differences which are as follows:- 

1. Number of xylem bundles are usually more than six (polyarch) in monocotyledon root.

2. Pith is large and we'll developed in monocotyledon root.

3. Xylem vessels appear circular or oval in T.S.

4. Monocotyledonous roots do not undergo any secondary growth.

Note- as the epiblema dies off (in old roots), a few outer layers of the cortex become suberized (mainly) or cutinized and form the exodermis. Exodermis is usually found in monocot roots.

Note: velamen:- this spongy tissues is found in areial roots or hanging roots of epiphytes (eg. Orchids- vanda).

1. It is an example of multilayered epidermis.

2. It absorbs atmospheric moisture by imbibition. 

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INTERNAL STRUCTURE OF YOUNG LEAVES

INTERNAL STRUCTURE OF LEAF

Generally leaves are divided into two categories - Dorsiventral leaves and isobilateral leaves. The differences in between them are as follows:

Dorsiventral or Bi-facial- Present at right angle to stem. Upper surface of leaf receives more sun light as compared to the lower surface, so there occur difference between internal structure of upper and lower surfaces.

Example - Dicots

Exceptions - Eucalyptus and Nerium

(leaves are isobilateral)

Iso-bilateral or Equifacial- Arranged parallel to stem. Both surfaces of leaf receive equal amount of sun light so no difference occurs between Internal structure of upper & lower surfaces.

Example: Monocots Exception - Lilium longiflorum (leaves are dorsiventral).

INTERNAL STRUCTURE OF DORSIVENTRAL LEAF

Cuticle is present on both surfaces but cuticle on upper surface is more thick.

In dorsiventral stomata are more on lower surface and stomata on upper surface are absent or less  in number.

The tissue between the upper & the lower epidermis is called the mesophyll. In dicot leaf, mesophyll is differentiated into palisade parenchyma (palisade mesophyll or palisade tissue) and spongy parenchyma (spongy mesophyll or spongy tissue).

Palisade tissue is situated towards the upper (adaxial or ventral) surface. It is made up of elongated cells. which are arranged vertically and parallel to each other and have more chloroplasts and a large vacuole.

Spongy tissue is situated towards lower (abaxial or dorsal) surface. The cells are oval or rounded and between the cells large air spaces/ air cavities are present.

INTERNAL STRUCTURE OF ISOBILATERAL LEAF

The thickness of cuticle is equal on both surfaces.

Usually stomata on both surfaces are equal in number.

Mesophyll is not differentiated into palisade and spongy tissues in isobilateral leases. Mesophyll cells have only a few intercellular spaces.

Note: 1. In isobilateral leaf, two distinct patches of sclerenchyma are present above and below the large vascular bundle and extend up to the upper and lower epidermal layers. These are called bundle sheath extensions.

2. In dorsiventral leaf, two distinct patches of parenchyma (mainly)/collenchyma are present above and below the large vascular bundle and extend up to the upper and lower epidermal layers. These are called bundle sheath extensions. Chloroplasts are absent in bundle sheath extensions.

VASCULAR BUNDLES OF LEAVES :

Similar types of vascular bundles are found in both dorsiventral and isobilateral leaves. Vascular bundles of leaves are conjoint collateral and closed.

Protoxylem is situated towards the adaxial (upper) surface and protophloem towards the abaxial (lower) surface in the vascular bundle. Leaves are devoid of endodermis and pericycle.

Vascular bundles are surrounded by a bundle sheath. It is made up of thick walled parenchyma.

Druses- crystal of calcium oxalate, star shaped e.g. Nerium.

Cystolith- Crystal of calcium carbonate, like bunch of grapes e.g. Ficus (Banyan, Rubber plant).

Raphides → Crystal of calcium oxalate, Needle shaped e.g. Eichhornia

Notes:- 

In the leaves of C4- plants (eg. sugarcane, maize etc.) bundle sheath is chlorenchymatous.

In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells. These are called bulliform cells or motor cells. When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.

The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.

The size of vascular bundles are dependent on the size of the veins. The veins vary in thickness in thickness in the reticulate venation of the dicot leaves.

The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins) as seen in vertical sections of the leaves.

Both upper & lower epidermis of Nerium leaves are multilayered. This is an adaptation to reduce transpiration.

Xerophytes with isobilateral leaves contain palisade tissue on both sides and few amount of spongy tissue is present in between palisade tissue.

Example :- Eucalyptus & Nerium.

1. Epistomatic leaf- stomata are present only on upper surface.

Floating leaves example- lotus (Nelumbium) Victoria regia, Nymphaea.

2. Hypostomatic leaf- stomata are present on lower surface. Mostly dicot leaves.

3. Amphistomatic leaf- stomata are present on both surfaces. Mostly monocot leaves.

4. Astomatic leaf- stomata are absent or non functional. Submerged leaves. Examples- Vallisneria, Hydrilla.

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INTERNAL STRUCTURE OF YOUNG STEM

Internal structure of young stem 

Internal stucture of dicot stem :

Internal structure of a typical dicot (dicotyledonous) stem shows following features :-

1. Epidermis : Epidermis is the outermost protective layer of the stem It is made up of elongated compactly arranged cells. It is single layered and lack of chloroplasts.

Stomata, multicellular hairs (trichomes) are present on epidermis. Thin cuticle is present on epidermis

Epidermis plays a significant role in protection

Trichomes help in preventing water loss due to transpiration

2. Cortex : The cells arranged in multiple layers helmen epidemic, and periack constitute the corte in dicotyledon stem cortex is divided into three parts.or sub zunes.

(a) Hypodermis

(b) Endodermis

(c) General cortex

 (a) Hypodermis :It is pattent lust below the epidermis. İs multilayered ihick . It is composed of collenchyma and cole contain chloroplasts.

(b) General Cortex. : This part is composed.of rounded the walled cells.of parenchyma conspicuous intercellular spaces Storage of food is the main function of the cortex.Resin canals/ mucilage casals are present in it. These are schlagenous in origin. The innermost layer.of the.cotex in.called endodermis.

(c) Endodermis : It is single layered The cells of endodermis ae rich in starch grains, thus lus also known as "starch sheath".

3. Pericycle Pericycle is situated below the endodermis. The pericycle of stem is multilayered.

In sunflower stem, pericycle is made of alternate bands of parenchymatous and sclerenchymatous cells. The part of pericycle which is present infront of the vascular bundle is made up of sclerenchyma and remaining part is composed of parenchyma. Part of pericycle which is situated in front of vascular bundle is known as Bundle cap. In sunflower stem, pericycle is heterogenous in nature Sclerenchymatous part of period is also known as Hard bast.

Note - Pericycle is present above the phloem in the form of semilunar patches of sclerenchyma.

4. Vascular Bundles : The wedge shaped vascular bundles are arranged in a ring. The ring arrangement of vascular hurdles is a characteristic of dicot stem. Each vascular bundle is conjoint, collateral and open and xylem is endarch.

5. Pith: This is well developed region, present in the centre. The cells of this region are made up of parenchyma.

6. Medullary rays : Radially arranged parenchymatous in between vascular bundles called pith rays or medullary rays. The main function of pith rays is radial conduction of food and water.

INTERNAL STRUCTURE OF CUCURBITA STEM:

It contains five ridges and five furrows. The vascular bundles are arranged in two rings. Each ring has five vascular bundles. In this way the total 10 vascular bundles are present

The vascular bundles of outer ring are small in size and situated below the ridges while the vascular bundles of inner ring are large in size and located below the furrows

Vascular bundles are conjoint, bicollateral and open and xylem is endarch. Outer cambium is functional.

Hypodermis is mainly present in ridge region.

General cortex is Sclerenchymatous.

Pericycle is sclerenchymatous.

INTERNAL STRUCTURE OF MONOCOTYLEDONOUS STEM

Epidermis: Epidermis is the outer most single celled thick laver. It is cOVered with thick cuticle Multicellular hairs are absent.

Hypodermis: Hypodermis of monocotyledon stem is made up of sclerenchyma. It is 2-3 layered thick, In monocot stem rigidity is more in hypodermis whereas in dicot stem elasticity is more. It provides mechanical support to the plant.

Ground tissue: It is large conspicuous parenchymatous. There is no differentiation of ground tissue in monocotyledon stem. It means ground tissue is not differentiated into general cortex, endodermis, pericycle, pith & medullary rays.

Note : Sometimes in some grasses, wheat etc. the central portion of ground tissue becomes hollow and is called pith cavity (pith cavity is found in stems of Cucurbita. Ricinus amongst dicots).

Vascular Bundle: Mary vascular bundles are found scattered in the ground tissue and V.B. are generally oval (egg shaped). Peripheral vascular bundles are generally smaller then the centrally located ones. Vascular bundles which are situated towards the centre are large in size and less in number. Vascular bundles which are situated towards the periphery are small in size but more in number. Each vascular bundle is conjoint, collateral and closed and xylem is endarch. Each vascular bundle is smanded by sclerenchymatous bundle sheath. So vascular bundles are called fibro vascular bundles. Water containing cavities are present within the vascular bundles.

(a)Xylem - In xylem number of vessels is less. In metaxylem there occurs two large vessels while in protoxylem there occurs one or two small vessels. Vessels are arranged in the form of V or Y. Just beneath protoxylem vessels, there occurs a water cavity which is schizolysigenous in origin. In which major part of water cavity is lysigenous in origin (formed due to lysis of protoxylem elements and few part of water cavity is schizogenous (formed by separation of cells).

(b)Phloem - It consists of sieve tube elements and companion cells. Phloem parenchyma is absent.

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Plant life cycles

Diversity in plant form in annuals, biennials, and perennials

Plants are unique and essential to life on earth. Unlike most living things, plants make their own food from sunlight and water. Either directly or indirectly, they are the primary food source for humans and other animals. Additionally, they provide fuel, replenish the earth's oxygen supply, prevent soil erosion, slow down wind movement, cool the atmosphere, provide wildlife habitat, supply medicinal compounds and beautify our surroundings.

Many plants are familiar to us, and we can identify and appreciate them based on their external structures. How ever, their internal structures and functions often are over looked. Understanding how plants grow and develop helps us capitalize on their usefulness and take the part of our everyday lives.

Vascular plants contain xylem and phloem which are the water, nutrient and food conducting tissues. Ferns  and seed-producing plants fall into this catagory

mono- cots (eg grasses) produce only one seed lkaf (cotyledon).

while dicots broadleaf plants) have two. The vascular systems, flow ers and leaves of the two types of plants also differ.These differences will be important in our discussion of plant growth and development.

Plant life cycles:-

 A plant is classified as either an annual, biennial or perennial based on its life cycle or how many years it takes to produce flowers and seeds.

The plant starts life as a seed, which germinates and grows into a plant.

The mature plant produces flowers, which are fertilised and produce seeds in a fruit or seed pod.

The plant eventually dies, leaving seeds which germinate to produce new plants.

ANNUAL-

An annual, such as a marigold, completes its life cycle in 1 year. Annuals go from seed to seed in I year or growing season.

During this period, they germinate, grow, mature, bloom. produce seeds and die.

Summer annuals complete their life cycle during spring and summer, most winter annuals complete their growing season during fall and winter. There are both winter and summer annual weeds, and understanding a weeds life cycle is important in controlling it. Of course, in most locations in Alaska the temperature does not allow for winter annuals. Some plants that are winter annuals in warmer climates act as summer annuals in Alaska.

BIENNIAL-

A biennial completes its life cycle in 2 years. During the first season, it produces vegetative structures (leaves) and food storage organs. The plant overwinters and then produces flowers, fruit and seeds during its second season. Swiss chard, carrots, beets, sweet William and parsley are examples of biennials.

Biennials can sometimes go from seed germination to seed production in one growing season. This phenomenon is referred to as bolting. It is a common occurrence in Alaska due to the midnight sun, or long day lengths. This is one reason why spin ach, radish and beets can be hard crops to grow in Alaska. These plants bolt instead of producing a good crop of leaves or roots. This situation can also occur when plants are exposed to extreme environmental conditions such as temperature variation and drought.

PERENNIAL-

Perennial plants live more than 2 years and are grouped into two categories:

herbaceous perennials and woody perennials.

Herbaceous perennials have soft, non woody stems that generally die back to the ground each winter. New stems grow from the plant's crown each spring. A delphinium is an example of an herbaceous plant. Trees and shrubs, on the other hand, have woody stems that withstand cold winter temperatures. They are referred to as woody perennials.

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TISSUE SYSTEM and VASCULAR BUNDLES

TISSUE SYSTEM

  

In higher plants several tissues work together in the form of a unit to perform a particular function. These tissues have the same origin. Such tissues form a system which is called tissue system. On the basis of location/position & structure / morphology tissues were categorised by Von Sachs (German scientist) into three types of tissue system. Each system usually consist of an association of tissues which perform specific function.

1. Epidermal/dermal tissue system. This system forms the outer most.covering of the whole plant body and comprises epidermis, stomata and epidermal appendages. e.g Root hairs, trichomes.

• Epidermis: It is outermost layer of the primary plant body and made up of elongated. compactly arranged cells. It is usually single layered and made up of parenchymatous cells Cuticle (Waxy thick layer) is present on the epidermis which prevents the loss of water. Cuticle is absent in roots Epidermal cells are with a small amount of cytoplasm lining the cell wall and a large vacuole.

• Trichomes:- The cells of the epidermis of dicot stem produce hairs called trichomes. The trichomes in the shoot system are usually multicellular. They may be branched or unbranched and soft or stiff. They may even be secretory called glandular hairs. The trichomes help in preventing water loss due to transpiration.

Note : In shoot system trichomes are usually multicellular.

Function: The trichomes help in reduction of water loss.

• Root hairs :- The root hairs are formed due to the elongation of the epidermal cells. These have a vacuolated protoplasm. The thin wall is made up of cellulose and pectic materials. Root hairs are always unicellular.

Function: Root hairs play an important role in absorbing water from the soil.

2. Ground tissue system: - It is the largest tissue system. All tissues except epidermis and vascular bundles constitute the ground tissue. It includes hypodermis, general cortex, endodermis, pericycle and medullary rays (pith rays), pith. In leaf G.T.S. consists of mesophyll. GTS is also called fundamental tissue system. G.TS. is made of simple tissues such as parenchyma, collenchyma & sclerenchyma. The GTS forms the main bulk of the plant.

3. Vascular/conducting tissue system:- 

The V.T.S. consists of complex tissues. xylem and phloem, It is also called specific tissue system.

Note : Primary structure of plant organ or primary plant body is mainly composed of parenchyma.

Types of development of primary xylem :

I. Centrifugal - In this type of development, the protoxylem is formed towards the centre (pith) and metaxylem is formed away from the centre, it means towards the periphery In this condition xylem is known as endarch ex. Stem of angiosperms & gymnosperms.

II. Centripetal - In this type of development protoxylem is formed towards the periphery near the pericycle and metaxylem is formed towards the centre (pith). In this condition xylem is called exarch ex. Roots.

III. Centrifugal and Centripetal :- Elements of metaxylem are formed on both sides of the elements of protoxylem. In this condition xylem is known as mesarch. ex Fern rhizome (underground stem).

VASCULAR BUNDLES

Xylem and phloem are collectively termed as vascular bundles. Which may or may not have cambium.

On the basis of arrangement of elements means location of xylem and phloem vascular bundles are divided into three categories.

Types of vascular bundles 

1. Radial vascular bundles When the xylem and phloem are present separately on different radio in alternate manner, then vascular bundles are called radial vascular bundles

In these vascular bundles xylem is exarch. The order of development of xylem in these vascular bundles is centripetal. Example : Most of the roots (Dicot, monocot, gymnosperm, fern root) 

Exception :- In Radish, carrot, turnip and sugar beet (Beet root) roots, conjoint, collateral vascular bundles are present.

2. Conjoint vascular bundles In the localar bundeswomand loan ton same radius of combine into a bundle. These are of two types :-

Collateral (Open) Bicollateral Conjoint collateral in this type of vascular bundle xylem and phloem re present on the same radius and phloem is present towards the periphery

These are two types :

(i) Open - the cambium is present between the xylem and phloem, then it is said to be open vascular bundle. Ex. stem of dicots/dicotyledons and gymnosperms

(ii) Closed- When cambium is absent between the xylem and phloem conjoint vascular buut then it ched closed vascular bundle.

Ex. Monocotyledonous stem and leaves of angiosperms.

In this type of vascular bundle. xylem is endarch and cool development of sum is centrifugal

(2) Conjoint, bicollateral and open vascular bundle -In this type of V.B two patches of phloem, one of each side of xylem and found. There are two strips of cambium (outer and innert), one oneach side of xylem are found. Only outer cambium is functinal.

Order of development of xylem is centrifugal means endarch condition is found.

ex.  stem of family cucurbitaceae and some plants of family solanaceae.

3. Concentric vascular bundles In these type of vascular bundies either xylem surrounds vascular bundles are always closed. They are of two  :

(a) Amphicribral or Hadrocentric In this type of vascular bundle xylem completely, Surrounded by phloem. It means xylem procent the centre of vascular bundle. Such types of vascular bundles are found in ferns rhizome (Underground stem). The order of development of xylem in ferns time is of both centripetal and centrifugal and we mesarch.

(b) Amphivasal or Leptocentric

• In this type of vascular bundle phlegm is completely surrounded by xylem. It means phloem is present centre of the vascular bundle.

• In this type of vascular bundle, xylem is endarch. stem of Dracaena, Yucca etc

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Secondary Growth

SECONDARY GROWTH

Secondary Growth:

• By the activity of laterel meristem (vascular cambium and cork cambium). Increase in the circumference girth/ thickness of the plant organs is called secondary growth.

• Normal secondary growth takes place in roots and stem of dicotyledons and gymnosperms.

• The tissues involved in secondary growth are two lateral Meristems: vascular cambium.and.cork.cambium.

• Secondary growth is not found in the leaves and monocots.

• Due to lack of cambium in monocotyledons, secondary growth is absent. But exceptionally secondary growth takes place in some monocotyledons. Such as-Palm, Date Palm, Coconut Palm, Yucca, Dracaena, Kingia, Sansevieria, Smilax, Agave etc These plants show abnormal secondary growth.

SECONDARY GROWTH IN DICOT STEM 

A. SECONDARY GROWTH IN STELAR REGION BY VASCULAR CAMBIUM

Secondary growth in stelar region begins earlier than the extrastelar region.

Formation of ring of vascular cambium- A Cambium which is present inside the vascular bundle (Means between primary xylem  meristem and primary phloem) is called intrafascicular cambium This is a type of primary lateral Meristems.

• First of all,cells of medullary  rays adjoining intrafasicular cambium become meristamatic to form interfascicular cambium which is a secondary lateral meristem.

intarafasicular and interfasicular cambium are collectively known  as vascular cambium or intra stelar Vascular cambium.

Vascular cambium is  formed in the form of a complete ring which made up of single layer of cells. 

In dicot stem, the vascular cambium is partly primary and partly secondary in origin.

Two types of cells are found in the ring of this vascular cambium.

1. Fusiform initials

2.Ray Initials

1.Fusiform initials are long with pointed ends, whereas ray initials are spherical/rounded or oval in shape. Fusiform initials are more in number in vascular cambium.

Activity of vascular cambium:

(a) Activity of fusiform initials:

Continuous perclinal divisions (parallel to longitudinal axis) takes place in fusiform then few cells are formed towards  the periphery and these cells are differenciated into secondary phloem or bast and the cells which are formed towards the  centre (towards pith) are differenciated into secondary xylem or wood.

The cambium is generally more active on the innerside than on the outer.

 Normally more secondary xylem is formed  as compared to the secondary phloem due to unequal distribution of hormones. (Secondary xylem is formed 8-10 times more as compared to the sea phloem).

Different stages of secondary growth in Dicotyledonae stem

By the pressure of secendary xylem, all the primary tissues such as primary xylem, pith are pushed towards  the centre.

The primary xylem however remain more or less intact in around the center. The primary phloem and earlier secondary phloem (old secondary phloem) get gradually crushed due to the continued formation and accumulation of secondary xylem.

Activity of Ray Initials :- Due to periclinal division ray initials cuts off (form) parenchymatous cells. These are called vascular rays (Xylem rays & phloem ray) or secondary medullary rays which passes through the secondary xylem and secondary phloem in the radial direction.They conduct water and food in radial direction. The order of development of vascular rays are both centripetal and centrifugal.

a) Formation of Annual Rings

• Annual rings are formed due to unequal activity of vascular cambium.

• The activity of cambium does not remain same. It is chargeable in the whole year

• Activity of vascular cambium is under the control of many physiological and environmental factors .
•  In temperate regions,the climatic conditions are not uniform through the year.

• In the spring season, the vascular cambium is very active and produces a large number of secondary xylem elements having vessels with wider cavities/lumens. The wood formed during this season is called spring wood or early wood.

• The spring wood is lighter in colour and has a lower density whereas the autumn (or winter) wood is darker and hes a higher density.

Note:-The autumn  and  spring wood are formed in the form of  concentric rings called growth rings.

• The two kinds of wood that appear as alternate concentric rings, constitute an annual ring.

• A ring of autumn wood and a ring of spring wood are collectively known as annual ring. The number of annual rings, formed in a tree give the idea of the age of the tree. The study of determination of age of a tree/plant by counting annuals rings is called Dendrochronology.

• A piece of wood start the stem up to in that region from the  base of stem with the help of Increment borer instrument. The annual rings are counted from that piece and again inserted (fitted) into the same stem as the same place.

• More distinct/clear annual rings are formed in that regions where climatic variations are sharp 

• More distinct annual rings are formed in temperate plants Because in temperate  regions, the climate conditions are not uniform  throughout the year.

• Distinct annual rings are not found in tropical plants. Distinct/clear annual rings are not formed in India except Himalayan regions (Shimla, Nainital ete).

• Least distinct annual rings are formed in seashore regions/coastal regions because the climate remains the same throughout the year.

• More clear annual rings are formed in deciduous plants as compared to evergreen plants .(In temperate region).

•  In deserts annual rings are less distinct.

• In annual rings bands of secondary xylem and xylem rays (Ray parenchyma) are present.

HEART WOOD & SAP WOOD:

In old trees the greater part of secondary xylem is dark brown.

The organic compounds like tannins, resins, gums oils and aromatic substances etc. are filled in lumen of tracheids and vessels of secondary xylem. Due to this, central region of secondary xylem becomes dark brown. It is called heart wood or duramen. These substances make it hard, durable and resistant to the attack of micro-organisms and insects. Heart wood comprises dead elements with highly lignified walls. Heart wood. provides mechanical strength to stem

• The peripheral region of secondary xylem which is light in colour, is called sap wood or alburnum.
• The function of sap wood is conduction of water and minerals.

• Heart wood does not conduct water because :

• Cavities of tracheids and vessels are progressively filled with waste materials.

The bladder/balloon like ingrowth of parenchyma cells enter in the lumen of vessels (mainly) & tracheids through the pits. Such bladder like ingrowths are called tyloses or tracheal plugs. Tyloses block the lumen of tracheary elements (vessels & tracheids).

Heart wood provides stiffness to the stem. The waste materials of heart wood are antiseptic in nature. Heart wood is resistant to the attacks of termites and insects and in rainy season it does not imbibe water. Thus it is the best quality of wood.

• Study of wood is known as Xylotomy. The wood is actually a secondary xylem.

• Position of youngest secondary phloem is just outside the vascular cambium.

• Position of oldest secondary phloem is just inside the primary phloem.

• Position of youngest layer of secondary xylem is just inside the vascular cambium.

• Position of oldest layer of secondary xylem is just outside the primary xylem.

• As the time passes amount of heart wood increases more as compared to sap wood.

Classification of wood:-

 A. On the basis of ammount of parenchyma wood is classified into two groups- 

1. Manoxylic wood- such type of wood contains more amount of living parenchyma. It is loose wood eg. Cycas.

2. Pycnoxylic wood- such wood contains less amount of living parenchyma.

Example- Pinus (conifers).

B. Classification based on vessels-

On the basis of presence and absence of vessels, wood is classified into two categories:-

1. Non- porous wood/Homoxylous wood- vessels are absent in this type of wood

 Example:- mostly gymnosperms.

2. Porous wood/ Heteroxylous wood- vessels are present in this type of wood .eg. mostly dicots (angiosperms) on the basis of arrangement of vessels porous wood is divided into two groups.

a. Ring porous wood- vessels are arranged in the rings in this type of wood.

Example:- in temperate region plants ex. Dalbergia.

b. Diffused porous wood- Asystematical distribution of vessels is found in this type of wood.

 Example:- in tropical region plants ex. Azadirachta ( neem).

Most durable wood - Tectona grandis (Teak= sagwan).

SECONDARY GROWTH IN EXTRA STELAR REGION BY CORK CAMBIUM

• Secondary  growth takes place in extra stelar region due to the activity of cork cambium.  Cork cambium is also known as phellogen or extrastelar cambium. the cells of the cork cambium are narrow, thin walled and nearly rectangular. Cork cambium develops usually in cortical region by hypodermis.

• As the stem continues to increase in girth due to activity of vascular cambium the outer cortical and epidermal layers get broken and need to be replaced to provide new protective cell layers. Hence sooner or later another meristamatic tissue called cork cambium or phellogen develops.

• Cork cambium is derived form the hypodermis (outer part of cortex) by dedifferentiation. Cork cambium is single or a couple of layers thick (mainly). It forms secondary tissues in extra stelar region.

• Cork cambium divides periclinally to form some cells towards the outside (towards epidermis) and some cells towards the inside (towards general cortex). Those cells which are formed towards outside become suberized. Due to this, these cells become dead. These dead cells are known as cork or phellem. Those cells which are formed towards the inside are differentiated into parenchyma and may contain chloroplast. These are called secondary cortex or phelloderm.

• Phellem, phellogen and phelloderm are collectively known as periderm.

• The cork is impervious to water due to suberin deposition in the cell wall. 

• Commercial cork is obtained from  Quercus suber (oak). Common bottle  Cork is made from this cork. 

• Due to activity of the cork cambium, pressure builds up on the remaining layers peripheral to phellogen and ultimately these layers dies and slough off.

• Ring of cork cambium remain living and active only for one year. each year, a new cork cambium is formed below the previous cambium. this new cambium derived from the secondary cortex or phelloderm.

• all the tissue which occur outside the innermost cork cambium are collectively termed as rhytidome. rhytidome include cork and tissue which become dead due to presence of cork. 

Lenticels : At certain regions, the phellogen (cork cambium) cuts off forms closely arranged parenchymatous cells on the outer side instead of cork cells. These thin walled, rounded, colourless, parenchymatous cells are called complementary cells. These cells are not suberized. As the complementary cells increase in number, pressure is exerted on the epidermis to which it ruptures, forming a lens-shaped openings called lenticels.

Complementary cells are formed by the activity of phellogen cork cambium.

• Lenticels are found in most of the woody trees. absent  in woody climbers.

• Lenticels are mainly found on woody stems and they are never found on leaves They are also present on some fruits.

•  Lenticels are not found in herbaceous dicots and monocot plants.

 Functions :

Exchange of gases  Lenticels permit the  Exchange of gases  between the outer atmosphere and the internal tissues of the stem. Main function 

Help in transpiration i.e. Lenticular transpiration.

BARK 

There are two views about the bark.

 Old view All the tissues situated outside the cork cambium are called bark. According to old view bark includes mainly dead tissues.

 Modem view Bark is non-technical term  that refers to all tissues exterior to vascular cambium, therefore including secondary phloem.  According to modern view bark includes both living and dead tissues. 

KIND OF BARK 

1.Ring Bark - Sheet bark continuous  Bark of equal thickening is called ring bark, It is formed around the stem in the form of a complete ring. When cork cambium in continuous then ring bark is formed. Example Bhojpatra (Betula utilis) A complete distinct ring bark is formed in this  plant. its bark was used as writing material/as a paper in ancient period, only cork layer was used. Ring bark is also formed in Eucalyptus.

2.Scaly Bark Discontinuous bark of unequal thickening is called scaly bark. This bark is formed around the stem in the form of pieces or fragments or patches.  When cork cambium is discontinuous then scaly bark is formed  eg. Guave (psidium guajava), Neem (Azadirachta indica), Mango (Magnifera indica), and Tamarind=Imli (Tamarindus) etc. Plants.

• Highly distinct scaly bark is fromed in Psidium guajava (Guava).

• Scaly bark is found in most of the woody plants.

• If bark is removed in the form of a ring (Girdling) from the base of main stem then root dies first due to lack of food.

• Girdling is not possible is monocot stem because vascular bundles are scattered.

• If complete bark is removed then plant dies due to excessive water loss.

• Bark that is formed early in the season is called early or soft bark. Towards the end of the season late or hard bark is formed.

• Secondary phloem and periderm are included in bark.

SECONDARY GROWTH IN DICOT ROOT

In the dicot root, the vascular cambium is completely secondary in origin.

• First of all the tissue located just below the phloem bundles means conjunctive tissue becomes meristematic. during the secondary growth in a dicotyledon root and forms separate curved strips of vasular cambium below the phloem bundles. Then after, the cells of pericycle is less  above the protoxylem also become meristematic to form additional strips of cambium. In this way a complete way a complete ring of vascular cambium is formed.

• The portion of vascular cambium which is formed by pericycle is less. The main portion of vascular cambium is formed by connective tissue.

• The shape of vascular cambium is wavy in the beginning, but later on it becomes circular due to the pressure of secondary xylem.

• The portion of vascular cambium formed by conjunctive tissue becomes meristematic first and forms the secondary xylem towards the centre. Ultimately the cambium becomes circular by the pressure of secondary xylem.

The activity of vascular cambium of root is the same as the activity of vascular cambium of stem. Secondary xylem is formed towards the innerside and secondary phloem is formed towards the outer side by vascular cambium. The portion of vascular cambium which is formed by pericycle is responsible for the formation of pith rays or medullary rays. These are made up of parenchyma. These pith rays are known as primary medullary rays (Multiseriate). A few medullary rays or pith rays are also formed from remaining Vascular cambium. These are called secondary medullary rays (uniseriate). Thus two types of medullary rays are found in the secondary structure of roots.

Note : Two types of medullary rays are formed in the dicot roots during secondary growth.The presence of two types of medullary rays is basic characteristic feature of roots. Only secondary medullary rays are formed in dicot stem during the secondary growth. Both of them conduct water and food in radial direction.

Cork cambium is developed from the pericycle in roots. Cork is formed towards the outside and secondary cortex is formed towards the inner side by the cork cambium. Lenticels are also found in roots but less in number as compared to stem. Cortex completely degenerates in roots after the secondary growth of one or two years. This falls down due to the pressure of cork, whereas in stem, it degenerates after the long duration.

1.Secondary growth is essential in roots to provide strength to the growing aerial parts of the plants and fulfill the requirement of water and minerals.

2.Generally clear annual rings are not seen in roots because roots are not effected by the changes of environment.

3.Secondary growth is not found in monocot roots.

In dicot roots, all cambia and pith rays (medullary rays) are secondary in origin.

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