44 



CELLULAR TISSUE. 



towards the passage of the future slit^. The free edges of the ridges of exit and 

 entrance correspond to the inner and outer edge of the original wall of division. The 

 origin of the respiratory cavity by separation of the sub-epidermal cells precedes the 

 formation of the slit. 



The mother-cell of the stoma and the products of its division are of equal height 

 with the other epidermal cells, and lie in the same plane as they. The subsequent 

 various unevenne>s of heiglit and position of epidermal- and subsidiary-cells and of 

 stomata arises through the growth of the cells subsequently to their division. During 

 growth all cells without exception increase in volume. But the passive tension by 

 the internal tissue, which the epidermis of growing, as also of adult parts undergoes, 

 finallv brings about, as Pfitzer has shown in the stomata of the Grasses '^j a considerable 



Fig. i8.— Ficus elastica; leaf, transverse section, e—e in each case the thickness of the epidermis ; W (600) upper 

 side, ^1 (390) under side;of the same very j'oung leaf; in Ax a mature stoma, which remains superficial, and a (tran- 

 sitor}-) hair; in A two cystolithcells, recognisable by their thickened outer wall, epidermal cells still undivided. 

 B (600) upper side, By (390) under side of a somewhat older leaf, epidermal cells dividing^. In B, x is a younger 

 cystolith-cell, and xi an older one which shows the peg-shaped outgrowth of the wall. C (390) older leaf, under side; 

 division of now three-layered epidermis is ended, stoma depressed, but the final size and form of the parts is not yet 

 attained. E upper side of a mature leaf, four-layered epidermis, cystolith-cell (375). 



diminution of the absolute height and breadth of the part of the stomatal cells which 

 border the slit. (In Zea Mais the breadth soon after the appearance of the slit amounts 

 to 11-4^, later to 1 1 -6 /.i, in the mature state to 5-4 /:i.) The same will apply for the other 

 cases above alluded to, in which the part of the guard-cells bordering the slit is narrower 

 and smaller than the connected ends. 



All the phenomena of development here touched upon are the same, whether the 

 epidermis consists of a single layer, or of several. Only in the latter case (Fig. 18) 



* Von Mohl, Verm. Schriften, pp. 254-257. 

 VII. /.f. 



-Strasburger, I.e. p. 308. — Pfitzer, Pringsheim'sjahrb. 

 - Pringsheim's Jahrb. \'II. I. c. 



