Anatomy of Plants

Epidermis.

The epidermis of the subaerial stems and leaves is primarily protective and usually consists of a single layer of living cells devoid of chloroplasts, and with thickened and cu ticularized outer walls, defensive to slight shock and small para sites, but permitting controlled evaporation. At intervals it is interrupted by small pores (stomata) leading from the outer air to the system of cortical intercellular spaces, and serving for gaseous interchange between the primary tissues and the outer air. Each stoma is surrounded by a pair of peculiarly modified epi dermal cells called guard cells which possess chlorophyll and open and close the pore, especially in accordance with the con ditions of transpiration. The stomata of leaves are often situated at the bottom of pits in the leaf-surface. Thus transpiration is checked by the creation of a still atmosphere in the pit above the pore. Such an arrangement of the stomata is found especially in plants which require to economize their water supply if tran spiration is in excess of absorption. In many plants living in con ditions which tend to promote excessive transpiration at certain hours of the day, the epidermis is doubled or trebled and forms a water tissue which supplies the immediate needs of transpira tion, and prevents the injury which would result in the epidermis if its cells were excessively depleted.

Hairs and Scales.—The epidermis of many species of vascular plants bears hairs or scales of varied form and function. Hairs are characteristic of stems and leaves of primitive pteridophytes and of many flowering plants, while scales characterize such pterido phytes as the advanced f erns. The simplest hairs consist of single elongated cells projecting above the general surface of the epidermis, while others consist of simple or branched cell-chains. The more complex scales are flat plates of cells commonly inserted on mounds or stalks, and may be placed parallel to the leaf-surface or projecting directly from it. By such emergencies transpiration is diminished by the stillness of the atmosphere between them. In other instances the hairs are glandular and excrete ethereal oils which tend to reduce transpiration, or they may be stinging, as in the common stinging nettle, in which the top of the hair is brittle, and when broken penetrates the skin and injects formic acid into the slight wound thus formed. In many cases both hairs and scales are shed before the stems and leaves are mature : in others they are retained throughout the life of the plant.

Hydathodes.—In many vascular plants special epidermal organs, known as hydathodes, are developed, especially on foliage leaves, and serve for the excretion of water in liquid form when transpi ration is diminished so that the pressure in the water-channels of the plant has come to exceed a certain limit. Hydathodes are widely distributed in plants in certain tropical climates under which active root-absorption continues while the air is nearly saturated with water-vapour. In some cases the hydathodes are specially modified single epidermal cells or are multicellular hairs without direct connection with the vascular system. The

cells concerned, like all excreting organs, have abundant proto plasm and large nuclei, and in many cases part of the cell-wall is perforated and resembles a filter. In others the hydathodes are associated with the ends of conductive strands and consist of groups of richly protoplasmic cells, as in the leaves of many ferns, while in flowering plants the hydathodes are most commonly directly connected with subjacent water-collecting cells known as epithem which act as intermediaries for the hydathodes and con ductive strands. To illustrate, water expelled through hyda thodes is often observable on the tips of grass leaves on a summer's evening and is commonly confounded with dew. Its expulsion as droplets is, however, due to the more rapid cooling of the air than of the soil as night approaches. Thus while active absorption continues by the roots, transpiration by the leaves is reduced in the evening and the conductive strands are over charged with water which is expelled by the hydathodes. There are many other types of glands such as nectaries, digestive glands, resin- and mucilage-glands, serving the most varied pur poses in plant-life. Some involve epidermal cells, while others are more deeply seated, but as they are not involved in the primary activities of the epidermis they are here passed over. It will, however, be apparent that the epidermis of the stem and leaf of a vascular plant may be highly specialized.

Epidermis of Root.—The epidermis of the root is funda mentally different from that of the stem or the leaf. In relation to its normal function of water-absorption its cell-walls are not cuticularized and usually remain thin. The absorbing surface is increased by many of the cells being produced into tortuous, delicate, unicellular tubes (root-hairs) which aid in anchoring the root, and readily absorb the surface films of water from the soil particles. A root-hair thus corresponds in function with a rhizoid of a bryophyte. The hairs are normally definitely re stricted to a more or less definite zone of the root a little removed from the growing root-tip. At the lower limit of this zone hairs are constantly being formed as the root advances in the soil, while at the upper limit they have passed maturity, are dying and are being destroyed. Thus beyond the zone which at any moment bears living absorbing hairs (the piliferous zone) the epidermis is extensively or completely destroyed, and its place is taken by the immediately subjacent cortical layer, which is not absorptive, but becomes protective like the epidermis of the stem or leaf. At the apex of the root, covering and protecting the delicate tissue of the growing point, is a special root-cap, consisting of a number of layers of cells produced by continual active local division of the apical epidermal cells, and which break down into mucilaginous products towards the outer surface of the cap, thus facilitating the passage of the apex as it pushes between the soil particles, and keeping the apex continually moist.