Is one of two main structural axes of a vascular plant




Дата канвертавання22.04.2016
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A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence (flowers), cones or other stems etc. The internodes distance one node from another. The term shoots is often confused with stems; shoots generally refer to new fresh plant growth and does include stems but also to other structures like leaves or flowers. The other main structural axis of plants is the root. In most plants stems are located above the soil surface but some plants have underground stems. A stem develops buds and shoots and usually grows above the ground. Inside the stem, materials move up and down the tissues of the transport system.

Stems have four main functions which are:

1. Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.

2. Transport of fluids between the roots and the shoots in the xylem and phloem.

3. Storage of nutrients.

4. The production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue.

Functions of the Plant Stem

The plant stem including its outermost bark has multiple uses ranging from logs, firewood, lumber, source of pulp for paper making, source of food, fiber, medicine, latex, tannin, dye and many more. It is also the most widely used part of the plant for asexual or vegetative propagation through various methods such as by stem cuttings , air layering or marcotting and other forms of layering, budding, and grafting.

In plant growth and development, the plant stem performs the following functions:

1. It supports the leaves, flowers and fruits and connects them with the roots. In trees and shrubs, the main stem or trunk provides a strong columnar structure from which branches are attached, raising the leaves upward to be exposed more fully to the sun.

2. It conducts water, nutrients and the products of photosynthesis to and from roots and leaves. It accommodates the transport system which is necessary for the vertical and lateral movement of water and sap within the plant body.

3. It helps store water, as in cacti, and the products of photosynthesis, as in the trunk of sago palm (Metroxylon sagu) and sweet palm (Arenga pinnata) which store large stock of starch;

4. Young green stem also performs a minor role in the production of food through the process of photosynthesis, but in some species (e.g. cactus) the stem is the chief photosynthesizing organ.

5. The plant stem serves as a means of asexual reproduction in many plant species.



  • LABORATORY work

Stem

1. Node: the part of a plant stem from which one or more leaves emerge, often forming a slight swelling or knob.

Why is it necessary to give this part of the stem a special name? Beyond the biological reason that there must be something special happening there in order for a leaf to “know” it must grow from this point, there is the convenience for us when we try to describe how leaves are arranged along a stem, which you will see when we look at leaf arrangement.

2. Internode: the part of a plant stem between two of the nodes from which leaves emerge.

I rarely use or see this term when trying to identify a plant, but it follows so logically from the definition of node that I thought it worthwhile to mention.

3. Bud: an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of the stem.

You’re probably already familiar with buds. In the illustration below, the bud is occurring in the “axil” of the leaf, which is the upper angle between the leaf stalk and the stem. Recognizing buds is important under two circumstances when trying to identify plants. 1) When you need to distinguish a bud from a “stipule” (the next term), and 2) When you need to determine whether a leaf is “simple” or “compound” (coming up!).

4. Stipule: a small leaflike appendage to a leaf, typically borne in pairs at the base of the leaf stalk.

The stipules shown here are more or less snug up against the stem and the leaf stalk. Stipules come in wildly varying forms, and not every plant even has stipules. They are good aids to identification, though, so look closely for them whenever you examine a new plant.



The plant stem is a component of the shoot system. Besides the stem, the plant shoot also consists of the leaves and the reproductive organs.

The plant stem has been described as a “central axis” to which all other parts are attached. In most plants the stems are conspicuous aboveground, but in some species they are hidden below the ground. Based on various criteria, there are other more classifications of the stem.

The first stem that develops from a seed arises from the epicotyl, an embryonic shoot within the seed.

LABORATORY WORK

NOTE: In grafting, as well as budding, the vascular cambium of the scion or bud must be aligned with the vascular cambium of rootstock. In woody plants the cambium is a very thin ribbon of actively dividing cells located just below the bark. The cambium produces conductive tissue for the actively growing plant (Figure 1). This vascular cambium initiates callus tissue at the graft and bud unions in addition to stimulating tissue growth on the basal ends of many vegetative cuttings before they have rooted.



Figure 1. Cross section of a woody plant stem.





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