In our previous examination of Aquatic Plant Anatomy we took a look at the stem and leaf.· In this third installment of Aquatic Plant Anatomy I am introducing the root.
Although the root is axial in construction it lacks the leaves of the shoot axis. Generally, roots develop in the soil, but they may also form in the water and in the air. Roots not only absorb water and minerals; they also provide the plants with firm anchorage in the soil. Occasionally the roots take over the function of a storehouse for nutrients.
When a seed germinates, the primary root or radicle (already present in the embryo) develops first. In a number of dicotyledonous plants the main root throws out subsidiary roots; these branch, become fibrous and eventually form the whole root system. Other dicotyledonous plants develop additional, adventitious roots in the axis of the shoot. Most aquarium plants belong to the latter category. The primary root often dies at a later stage. Monocotyledons always have a short-lived primary root which is replaced by adventitious roots. Ferns have adventitious roots only.
For the aquarist, it is invaluable that the plants have the habit of forming adventitious roots. Thus various methods of asexual propagation are possible, e.g. the use of cuttings for growing new plants. All roots growing from newly planted rhizomes or nodules develop adventitious roots. Buds form on the roots of some plants, e.g. Microsorium and may develop into adventitious plants.
The root grows by means of a vegetation cone protected by a special covering—the root cap (Il. 1). The younger sections of the root are enclosed by a one-layered rhizodermis (epidermis of the root). It differs from the epidermis of the shoot axis and the leaves in that it has very thin cell walls and lacks a cuticular layer as well as any stomates. Slightly behind the vegetation cone root hairs develop; they embed themselves in the soil surrounding them and support water and mineral absorption. Rhizodermis and root hairs die at a fairly early stage to be replaced by a cortical layer formed from cells which have become suberose. The cortical layer consists predominantly of fundamental tissue cells; in marsh and aquatic plants it may be crossed by ventilation ducts (Ill. 1.2).
The skin of the vertically upwards-growing respiratory roots of many Ludwigia species is made up almost entirely of spongy respiratory tissue (Ill. 1.4). In addition,· central cylinder is situated immediately below the epidermis of the root. Lateral roots form in the outermost layer of cells of the central cylinder and branch off. Thus, they develop in direct contrast to lateral shoots, lateral roots having to push their way through the epidermis of the root (Ill. 1). The central cylinder is mainly composed of the vascular bundle of the root system. . It may be very reduced in plants with a permanently submerged habit (Ill. 1).
Some aquatic plants, e.g. Ceratophyllum, Utricularia and Wolffia are rootless.
Radicle: The first part of a seedling (a growing plant embryo) to emerge from the seed during the process of germination. The radicle is the embryonic root of the plant, and grows downward in the soil
Adventitious roots: A root that grows from somewhere other than the primary root, for example, roots that arise from stems or leaves.
Cuticular layer: Waxy covering produced by the epidermal cells of leaves, young shoots and all other aerial plant organs
Suberose: Having the appearance of being gnawed
Rhizome: Horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes
Adventitious: Growing from an unusual part of the plant