128 



THE BIOLOO T OF A PLANT. 



the upper part of the leaf, as might be expected from their 

 functions in connection with the action of light (see page 147). 



The epidermis, or skin of the leaf, consists of translucent, 

 greatly flattened cells having peculiar wavy outlines and rela- 

 tively thick walls (Figs. 58-61). Upon 



A- ^\ y-^" rn ' c ' tne veins they become elongated, and 

 their walls are considerably thickened, 

 especially upon the midrib (Fig. 58, 

 They generally contain large, distinct 

 nuclei, and often considerable proto- 



i.c. n a! ? plasm. The wavy epidermal cells, 



particularly in young plants, contain 



^v \ ^^rCt^ some chlorophyll and starch, though 



in this respect the feni is somewhat 

 exceptional. 



In the rhizome the epidermis forms 



g f '**~\) { a continuous layer over the whole sur- 



FIG. ei. (After Sachs.)-Epi- face. In the leaf, however, this is not 



dermal cells of Pteris flaM- ft c t | ie e pj(] erm i 8 O11 the lower 



Ma, showing the development 



of stomata. A, very young side being perforated by holes leading 



mother-cell; s.r, sudsidiary O r stomata (singular, stoma) (Fig. 61). 



* These holes do not pass into the cells, 



but are gaps or breaks between certain cells of the epidermis, 

 and open directly into the intercellular spaces, of which they are, 

 in fact, the ends. That portion of the intercellular labyrinth 

 which directly underlies the stoma is sometimes called the respira- 

 tory cavity. Each stoma is bounded, as in most plants, by two 

 curving guard-cells, which are generally nucleated, and, unlike 

 epidermal cells generally, contain abundant chlorophyll-bodies 

 and starch. 



The guard-cells are capable of changing their form accord- 

 ing to the amount of light, the hygroscopic state of the atmos- 

 phere, and other circumstances, and thus open or close the hole 

 or stoma between them. This action is of great importance in, 

 the physiology of the plant (transpiration, p. 147). 



In Pteris cretica and P. flabellata the stomata develop as follows : A 

 young epidermnl cell is divided by a curved partition into two cells, one of 

 which (Fig. 61) is called the initial cell of the stoma (e.c). This is again 



