Plant form

Most plant species at first sight appear very similar since all four organs, the root, stem, leaf and flower, are present in approximately the same form and have the same major functions. The generalized plant form for a dicotyledonous and a monocotyledonous plant can be seen in Figure 5.2. Flowers are the site of sexual reproduction in plants and their external appearance depends principally on the agents of pollination.

Figure 5.2 Generalized plant form : (a) monocotyledon; (b) dicotyledon

Roots
Root morphology (see Figure 5.3). The function of the root system is to take up water and mineral nutrients from the growing medium and to anchor the plant in that medium. Its major function involves making contact with the water in the growing medium. To achieve this it must have as large a surface area as possible. The root surface near to the tip where growth occurs (cell division in the meristem) is protected by the root cap. The root zone behind the root tip has tiny projections called root hairs reaching numbers of 200–400 per square millimetre, which greatly increase the surface area in this region (see Figure 5.4). Plants grown in hydroculture, e.g. NFT, produce considerably fewer root hairs. The loss of root hairs during transplanting can check plant growth considerably, and the hairs can be points of entry of diseases such as club root. Figure 5.4 shows that the layer with the root hairs, the epidermis, is comparable with the epidermis of the stem (see stem structure); it is a single layer of cells which has a protective as well as an absorptive function.

Two types of root system are produced; a taproot is a single large root which usually maintains a direction of growth in response to gravity (see geotropism) with many small lateral roots growing from it, e.g. in chrysanthemums, brassicas, dock. In contrast, a fibrous root system consists of many roots growing out from the base of the stem, as in grasses and groundsel (see Figure 5.3).
Stems
The stem's function is physically to support the leaves and flowers, and to transport water, minerals and food between roots, leaves and flowers (see stem structure). The leaf joins the stem at the node and has in its angle (axil) with the stem an axillary bud, which may grow out to produce a lateral shoot. The distance between one node and the next is termed the internode. In order to perform these functions, the stem produces tissues specially formed for efficiency. It must also maintain a high water content to maintain turgor.

Buds
A bud is a condensed stem which is very short and has small folded leaves attached, both enclosing and protecting it. On the outside the leaves are often thicker and dark to resist drying and damage from animals and disease. A meristem is present at the tip of the stem, from which a flower or vegetative growth will emerge. A terminal bud is present at the tip of a main stem and will grow out to increase its length. Where leaves join the stem, axillary buds may grow into lateral shoots, or may remain dormant.

Figure 5.5 Structure of a bud : (a) Brussels sprout and (b) magnified image

Leaves
The leaf, consisting of the leaf blade (lamina) and stalk (petiole), carries out photosynthesis, its shape and arrangement on the stem depend on the water and light energy supply in the species' habitat. The arrangement of leaves and examples in different species, along with the major differences between monocotyledons and dicotyledons. Leaf structure, as an organ of photosynthesis.