DIV. I 



MORPHOLOGY 



51 



rigidity of the epidermal cells. On the other hand, the absence of 

 cuticle from the root facilitates the absorption of water and salts from 

 the soil. Deposits of wax are also present in the cutinised layers of the 

 epidermis, and consequently water will now off the epidermis without 

 wetting it. The wax is sometimes spread over the surface of the 

 cuticle as a wax covering. This is the case in most fruits, where, 

 as is so noticeable on plums, it forms the so-called bloom.. The 

 wax coverings may consist of grains, small rods (Fig. 44), or crusts, 

 soluble in ether or hot alcohol. 



The epidermis may not only protect the more internal tissues from loss of water 

 by hindering evaporation, but also by serving as a place of storage of water. The 

 unthickened lateral walls of these cells become folded as the water is withdrawn 



FIG. 45. Epidermis from the under side of a leaf of Tradescantia virginica. A, In surface view. 

 5, in transverse section ; I, colourless rudiments of chroma tophores surrounding the nucleus, 

 (x 240. After STRASBURGER.) 



from the cavity and stretch when the cell becomes again filled. Such an epidermis 

 is frequently also composed of several layers of cells. 



The mechanical strength of the outer walls of epidermal cells is increased in 

 some plants by the deposition of calcium carbonate or of silicic acid. In the case 

 of Equisetum the silicification is so great that the tissues are used in polishing tin- 

 ware. The pericarp of the Grass, Coix lachryma, is almost as hard as the opal. 



The epidermis of fruits, and particularly of seeds, exhibits a considerable variety 

 of modifications in its mode of thickening and in the relations the thickening 

 layers bear to one another. The purpose of these modifications in the epidermis 

 becomes at once evident when it is taken into consideration that, in addition to 

 protecting and enclosing the internal parts, the epidermis has often to provide for 

 the dissemination and permanent lodgment of the fruits and seeds. 



Among the ordinary cells of the epidermis there occur as a rule 

 stomata and hairs which are especially characteristic of this tissue. 



2. STOMATAL APPARATUS ( 41 ). The presence of stomata in the 

 epidermis is characteristic of most parts of the more highly organised 

 plants that are exposed to the air. Each STOMA is an intercellular 

 passage or pore bounded by a pair of curved, elliptical or half-moon- 

 shaped cells called GUARD CELLS. The pore and guard cells together 



