THE ANGIOSPERMAE : STEMS 869 



formed by the cellulose outer walls of the cells, more or less impregnated with 

 cutin. It is sharply separated from the true cellulose wall next to the cell 

 lumen. The cutinized layer is not invariably present, but when it occurs 

 it is frequently much thicker than the true cuticle and may be the chief 

 protective layer. It often accompanies the cuticle in forming prominent 

 wedges between the radial walls of the cells (see Fig. 445). 



Beneath the epidermis there is often, in plants of dry or exposed habitats, 

 a second, or even a third, specialized layer, the walls of which are not cutinized 

 but may be greatly thickened with cellulose or lignin, or else may be quite 

 unthickened. It may augment both the protective and the water storage 

 functions of the epidermis, and it is referred to either separately as the hypo- 

 dermis, or both layers together may be termed a multiple epidermis. 

 Strictly speaking, the latter name should not be used unless it is known 

 that both layers of cells have originated from the dermatogen by tangential 

 cell divisions. As this cannot often be proved it is best to retain the general 

 term hypodermis. 



The epidermis of young, green stems contains a number of stomata, 

 which resemble those on the leaves. Their structure will be fully described 

 in Chapter XXII. They are not normally so numerous as on the leaves, 

 but in plants with reduced or abortive leaves they may be the only means 

 for gas exchanges with the atmosphere, and in such cases they have the same 

 importance in photosynthesis as those on normal leaves. 



2. Epidermal Oiitgrozvths. Outgrowths of the epidermis take many forms 

 and are included under the general term trichomes, which covers not only 

 true hairs, but other modified structures such as glands and prickles. Their 

 typical forms have already been described in Chapter XIII. They are, of 

 course, not confined to the stem, but no distinction can be drawn between 

 organs in this respect, since identical trichomes may be produced at all points 

 on the surface of the shoot. Their distribution is, however, often restricted 

 to definite lines or areas and may be a mark of distinction between related 



species. 



Despite the immense variety among trichomes, a complete anatomical 

 series may be traced between the simplest, which are merely prolongations 

 of a single epidermal cell, and massive structures which arise from groups 

 of cells and involve also the sub-epidermal tissues and may even receive one 

 or more vascular bundles. These extreme cases have sometimes been 

 distinguished as emergences, but in spite of their apparent differences no 

 sharp line can be drawn between them and the simpler trichomes. 



Hairs can be classified under three main types: simple, branched, and 

 peltate. Simple and branched hairs may consist of one or of many cells. 

 The branched types are sometimes quite elaborate, like small bushes ; or they 

 may be flattened, the branches forming a rosette around a short stalk-cell. 

 The latter type is related to the peltate or scale hairs, which are similar to 

 flattened branched hairs but with the branches cohering into a disc. These 

 discs, if they overlap, as in the Eleagnaceae, form a complete protective armour 

 over the surface of the epidermis. 



