TISSUE SYSTEMS 



331 



Upon making a vertical section through a stoma we usually 

 find that the guard-cells are placed nearly or quite on a level 

 with the rest of the epidermis. In some oases, however, and 

 especially when situated upon leaves of a leathery or hardened 

 texture, the stomatal cells are depressed below the others ; in 

 some rare instances, again, they are elevated above them. 



The stomata vary in form and position in different plants, 

 and in different parts of the same plant, but they are always the 

 same in any particular part of plant. They are usually placed 

 singly upon the epidermis, at regular [fig. 704) or irregular 

 intervals. In Banlsia and some other plants we find little 

 cavities in the under surface of the leaves which contain a 

 number of hairs {fig. 703, b), and between the latter very small 

 stomata. 



Fig. 710. In these figures the development of the stoma of I/ijadnthvs 

 orieftfalis is represented from the firsc division of the mother-cell in a into 

 two daugliter-eclls, to the complete separp,tion shown in Ji. p, p. Paren- 

 chyma of the leaf, e, (?. Epidermis cells, "'f^. Stoma. /.Air-cavity. After 

 Sachs. 



The distribution of stomata over the surface of the sporophyte 

 varies very much. They are found especially upon leaves, more 

 particularly on their under surface. On the floating leaves of 

 water plants, as in the Water-lily, however, we find them only 

 on the upper surface ; while in vertical leaves the stomata are 

 equally distributed on the two surfaces. They occur also oa the 

 >oung green stem and branches of plants, and on parts of the 

 flower. In those plants which have no foliage leaves, as the 

 Caidacvce, they abound upon the green succulent stems. They 

 are absent, as a rule, from roots and all submersed parts of 

 plants. 



The number of stomata varies considerably. The following 

 table will give some idea of their abundance in leaves, and it will 

 be observed that the number of stomata is usually greatest 



