Plant Anatomy: Part 1 The Stem
Plant anatomy, or "morphology", refers to the description of the structure and parts of a plant.· Apart from the few simple mosses, aquatic plants consist of three basic organic parts: axis of the shoot or stem, leaf and root. Stem and leaves may be termed comprehensively as 'shoot'. Both differentiate from the shoot vegetation cone which they have in common. In this article I am introducing the first part, the stem or axis of the shoot.
Functions of the Stem
The stem develops either in the air, or in water or in the soil. Its growth is vertical to horizontal it can be the main axis or a branch. 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.
Stems are composed of nodes, this hold buds which grow into one or more leaves, flowers, or other stems, Internodes, the segment of the stem between two nodes. (ill.1)
Plant Shape and Internodes
It is mainly the length of the internodes which determines the habit of aquatic plants (Ill. 1). With elongated internodes the stem is clearly visible and the leaves are positioned at some distance from each other. Hygrophila polysperma is an example of this type. With compressed internodes, the stem is not visible as its base is concealed by densely positioned leaves, e.g. Echinodorus and Cryptocoryne species. The length of the internodes influences the form of the plant.(ill.2)
Parts of the Stem
The Stem usually consist of three tissues, dermal tissue, ground tissue and vascular tissue.
The dermal tissue covers the outer surface of the stem, in aquatic plants it protects and control gas exchange. There is a thin (or no) cuticle, plant cuticles are a protective waxy covering produced only by the epidermal cells. The primary function of cuticles is to prevent water loss, thus most hydrophytes have no need for cuticles. The ground tissue usually consists mainly of parenchyma cells and fills in around the vascular tissue. It sometimes functions in photosynthesis. The vascular tissue provides long distance transport and structural support.
The angle between the leaf and the stem is called leaf axil. (ill.1). Potentially, each leaf axil is capable of producing a bud which may grow into a lateral shoot, leading to branching of the mother shoot. The degree of branching varies greatly from one species to another. As a rule, aquatic plants with elongated stems branch quite regularly thus those which are rooted in the bottom develop a bushy growth while the floating species grow into a dense mat. Aquatic plants with a compressed stem rarely develop lateral shoots.
Stem Branching and Flowering
Two different types of stem branching exist. In one type the top of the stem (vegetation cone) continues its growth provided it is not disturbed; all lateral shoots are then subordinate to the mother shoot, and flowers and inflorescences appear at the side of leaf axils. Flowering Ludwigia species is a good example(Ill.3).
In the other type, the vegetation cone of a shoot stops growing after some time; it either dies or loses strength after producing one flower or one interflorescence. The mother shoot is replaced and overgrown by a lateral shoot. Flowering Cryptocoryne species exemplify this type of branching (Ill. 4).
The vegetation cone (also Apex) is the length at the top part of the stem where growth carries out itself. In that part and at some distance from the tip undifferentiated divisible cells reach a state of permanency. They take over certain functions, each of them being determined by their position in the stem.
In the inner area of a stem there are bundles of conducting tissues called vascular tissues these are composed of more than one cell type and carry the water, organic and inorganic substances (ill.5). Two main types of vascular tissue exist, the xylem and phloem. These two tissues extend from the leaves to the roots, and are vital conduits for transporting water and minerals from roots and sugar from leaves.
Cross Section of a Stem
The stem is a complex structure composed of several chambers and parts. Vascular tissue (dotted) and aeration ducts are shown in the cross section of submerged shoot axes. The following describes the main parts of stem
Apical (terminal) bud: The primary growing point located at the apex (tip) of the stem.
Axillary ( lateral) bud: A bud that develops in the axil of a leaf.
Node: Region of stem from which leaves, or branches arise.
Internodes: A region of stem between nodes.
Leaf Scar: A mark indicating the precedent place of attachment of petiole or leaf base.
Stipular Scar: A mark indicating former place of attachment of stipule.
Lenticel: A pore in the bark.
Prickle: A sharp pointed outgrowth from the epidermis or cortex.
Terminal Bud Scale Scar Rings: A ring indicating the previous place of attachment of bud scale
Vascular Bundle or Trace Scar: A mark showing former place of attachment within the leaf scar of the vascular bundle.
It is important to understand the anatomy of our aquatic plants to perfect our aquarium aquascaping.· A solid grasp on how plants grow can effectively help the aquascapers trim and train plants to grow in a fashion that not only fits the aquascape but enhances the beauty of the aquatic plant.
References
www.cactus-art.biz/note-book/Dictionary
The Complete Guide to Water Plants - Muhlberg
