CORTICAL SYSTEM. - STOMATA. 519 



then either in two pairs, as in Ficus elastica, or the four cells form the 

 quadrants of a circle, as in various Proteaceae. 



Stomata are most abundant usually on the lower surface of leaves, often 

 wanting on the upper surface except on the floating leaves of aquatic 

 plants, where they exist on the upper surface, and are absent where the 

 leaf touches the water. They are occasionally found in the interior of 

 organs, as on the replum of Crucifers. They vary in frequency, partly 

 "bearing proportion to the size of the cells of the epidermis, partly irrela- 

 tive to this. Sometimes 100 will be found in a square line, sometimes as 

 many as 1000 to 3000. On the leaf of Brassica Rapa a square line bears 

 1800 on the upper face, 3500 on the lower ; Victoria reyia 1800 on a 

 square line above, and none below. A few other examples may be cited. 



On the lower face. 



Cherry-Laurel .............. None. 625 to a square line. 



Laurustinus ................ do. 625 



Daphne Mezereum ............ do. 30 ,, 



Carnation .................. 250 250 



Garden Flag ................ 80 80 



Garden Rhubarb ............ 7 30 



Lilac ...................... None. 1000 



From the researches of Duchartre, Morren, and others, the following 

 conclusions may be drawn, subject, however, to many exceptions. Sto- 

 mata are more abundant in woody than in herbaceous plants, in leathery 

 leaves rather than in those of thinner texture. Succulent leaves contain 

 the smallest numbers of stomata. Where leaves are alike in texture and 

 colour on both surfaces, the number of stomata is about equal on both 

 sides ; when one side is glossy and the other dull, the stomata are most 

 abundant on the latter, &c. 



Form of Epidermal Cells. The cells of the epidermis exhibit a great 

 variety of forms in the leaves and petals of Phanerogamia. It is very 

 common for the side-walls, by which they adjoin, to be sinuous or zig- 

 zagged, often presenting very elegant patterns (fig. 565), especially on 

 petals. The external wall of the cells is usually more or less convex ; 

 and in petals this condition is carried further, through numerous grada- 

 tions, until we find a papillose condition, arising from each epidermal cell 

 being produced above into a little obtuse cone. 



Hairs ; Trichomes. Hairs and scales of all kinds, " scurf," 

 such as we see in the Bromeliacese &c., depend on the development 

 of the epidermal cells. Simple hairs are merely single epidermal 

 cells produced into a tubular filament ; cell-multiplication usually 

 occurs in such hairs, so that they present a number of joints (fig. 

 567, b) ; and not unfrequently they are more or less branched (fig. 

 567, c, cl). Glandular hairs differ merely in certain of their cells 

 secreting oils or resins in their cavities (fig. 567, /.) Scales are 

 produced by epidermal cells growing out into flat cellular plates 

 instead of projecting filaments. Thorns, such as those of the 

 Hose, the prickles of leaves, like those of the Holly, &c., are epi- 



