STEM

Stem develops from the rule of the embryo of germinating seed. The stem bears nodes and internodes. The region of the stem where leaves are bone are called nodes while the portions between two nodes are called internodes. Stem shows negatively geotropic growth. Stems, support buds and leaves and serve as conduits for carrying water, minerals and food (photosynthetic). Stems place the leaves in favorable positions for exposure to light. The vascular system inside the stem forms a continuous pathway from the root through the stem, and finally to the leaves It is through this system that water and food products move.

Forms of stem :

(1)Caudex/Columnar :- it is unbranched,

erect, cylindrical stem and marked with scars of fallen leaves.

eg - Palmi.

(2) Culm :- Jointed stem with sülid nodes & hollow internodes eg :- Bartaco Gramineae

Modification of stems :

A-sub-aerial modification of stem. These are creeping stems.

(1) Runner - In these stems roots develop at lower side and leaves at upper side from the nodes eg Cynodon dactylon (doob grass), Oxalis, Strawberry.

(2) Stolon - In it branches develop from the lower part of the main stem and grow for some distance like arch and finally touch the ground to give rise to new shoot Eg. Fragaria (Wild strawberry), jasmine. peppermint 

(3) Sucker - In it the main stem grows in the soll but branches deselop bra basal mad underaround nodes and comesout from the soil. Eg Mint, pineapple, Chrysanthemum, banana.

(4) Offset - Generally these occur in aquatic plants which have fragile stem. Internodes of offset ate small & mid each node bears a rosette of leaves and a tuft of roots It is ako known as aquatic runner. Eg Pistia Eichhomia.

B - Underground modification of stem

This type of modification occurs generally for food storage and vegetative propagation

(1) Tuber - The tips of branches become swollen in the soil. Eyes are found on tuber which are axillary buds and axillary buds are covered with scaly leaves. Eg. Potato

(2) Rhizome - It is fleshy stem which grows horizontally in the soil. Nodes and small internodes are found which are covered by scaly leaves. Eg. Ginger, turmeric, canna, water lily, banana.

(3) Corm - It is condensed structure which grows vertically under the soil surface. Eg. Colocasia, Alocasia, zaminkand, saffron, Colchicum > Lillace

Organ of perennation - Underground stems of potato, ginger. turmeric. Colocasia, zaminkand are modified to store food in them. They also act as organ of perennation to tide over conditions unfavourable for growth.

(4) Bulb - Stem is highly reduced and disc like and surrounded by numerous fleshy leaves. Many roots arise from its base. Eg. Onion, garlic. The fleshy leaves of onion and garlic store food.

Note : Type of stem in banana is rhizome and modification is sucker. Banana propagates through thiome. Aerial part of banana plant which looks like stem is pseudostem (leaf bases).

C- Aerial modification of stem

(1) Stem tendril - In this type of modification axillary bud forms tendril in place of branches aıd helps in climbing of those plants which have weak stem. Eg. Grapes/Grape Vines. Passiflora, gourds (cucumber, pumpkins, watermelon).

(2)Stem thorn - Thom develops from axillary bud of the stem. It may bear leaves, flowers

Eg. Carissa (karonda), Bougainvillea pomegranate, Citrus. 

• Thom protects plant from browsing animals It is a woody pointed structure.

(3) Phylloclade - Stem is modified into a fleshy flat (Opuntia) or fleshy cylindrical (Euphorbia) and green leaf like structure and carries out photosynthesis like leaf The leaves are modified into spines Eg. Opuntia, Euphorbia, cactus, Casuarina (cylindrical).

Structure

Vascular system The vascular system consists of xylem, phloem and vascular cambium. It can be thought of as a plant's plumbing. Xylem tubes conduct water and dissolved mineral also, phloem tubes carry food such as sugars.

The cambium is a layer of meristematic tissue that separates the xylem and phloem and continuously produces new xylem and phloem cells. This new tissue is responsible for a stem's increase in girth.

The vascular cambium is important to gardeners in several ways. Xylem and phloem tissues on a grafted scion (top wood) and rootstock need to line up, if the graft is to take. Careless weed trimming can strip the bark off a tree, injuring the cambium and girdling the tree and causing it to die. This often occurs when string weed trimmers are used to remove grass from around the trunk of trees. Stem boring insects are attracted to cuts made by weed trimmers.

The vascular systems of monocots and dicots differ. Although both contain xylem and phloem, these structures are arranged differently in each. In a monocot, the xylem and phloem are paired in bundles dispersed throughout the stem. The vascular system in dicots is said to be continuous because it forms rings inside the stem. The phloem forms the outer ring and eventually becomes part of the bark in mature woody stems. The xylem forms the inner ring. Sapwood is newer xylem that is still conducting and heartwood is older xylem that is crushed in the center of the stem and is no longer conducting, Heartwood is filled with waste materials such as gums, resins tannins and oils.

The difference in the vascular systems of monocots and dicots is of practical inter est to gardeners because some herbicides affect only one group. For example, 2, 4-D kills only plants with a continuous vascular system (dicots). Non selective herbicides, on the other hand (eg.. glyphosate), kill plants regardless of their type of vascular system.

Nodes:  A node is an area on a stem where buds are located. It is a site of great cellular activity and growth where small buds develop into leaves, stems or flowers. When pruning, it is important to locate a plant's nodes. Generally, you want to cut just above, but not too close to, a node. Pruning in this manner encourages the buds at that node to begin development and ultimately form new stems or leaves.

The area between nodes is called the internode. Its length depends on many factors, including genetics. Other factors also can influence internode length:

• Reduced soil fertility decreases inter node length, while an application of high-nitrogen fertilizer can increase it.
• Lack of light increases internode length causing spindly stems. This situation is known as stretch, or etiolation, and often occurs in seedlings started indoors and in houseplants under low light conditions. Internode length varies seasonally.
• Early-season growth has long inter nodes; late-season growth has shorter internodes.
• Plant energy divided among three or four side stems, or diverted into fruit growth and development, shortens inter node length.
• Plant growth regulator substances and herbicides also can influence internode length.

Types of plants and their stems:

fig: Types of plants and their stems

 Trees generally have one main trunk, usually more than 12 feet tall when mature. In contrast, shrubs have several main stems, less than 12 feet tall when mature.

Most fruit trees, ornamental trees and shrubs have woody stems. These stems con tain relatively large amounts of hardened xylem tissue (heartwood) in the central core. The sapwood is the light colored living xylem near the outside just under the bark of a tree.

Herbaceous or succulent stems contain only a little xylem tissue and usually live for only one growing season. In perennial plants, new herbaceous stems develop from the crown (root-stem interface) each year.

Canes are stems with relatively large pith or central strength-giving tissues. They usually live only 1 or 2 years.

Examples of plants with canes include roses, blackberries and raspberries. For fruit production, it is important to know which canes to prune. how to prune them, and when to prune them. For example, two-year old raspberry canes are pruned at the ground. One-year old canes are left to produce the following Season.

A vine is a plant with long, trailing stems. Some vines grow along the ground, while others must be supported by another plant or structure. Twining vines circle a structure for support. Some circle clockwise (e.g. hops and honeysuckle), while others circle counterclockwise (e.g., pole beans). Climb ing vines are supported either by aerial roots (e.g., English ivy and poison ivy), by slender tendrils that encircle a supporting object (e.g., cucumbers, gourds, grapes and passion flowers) or by tendrils with adhesive tips (e.g., Virginia and Japanese creeper).

Stems as food :

The edible tuber of a potato and the Jerusalem artichoke are both fleshy under ground stems. Asparagus and kohlrabi are enlarged, succulent stems. A ginger "root" is actually a rhizome or underground stem. Sugarcane and bamboo shoots are stems.

The main function of the stem is spreading out branches bearing leaves, flowers and fruits. it conducts water, minerals, photosynthates (food). some stems performs the function of storage of food (potato, ginger, zaminkand and colocasia,) support, protection and of vegetative propagation.

THANK YOU FOR VISITING