92 BOTANY. 



horizontal division tlie several superimposed circular rows* 

 of cells are formed. 



120. — In true mosses the sporangia possess an epidermal 

 system Wihich is composed of a layer of strongly cuticular- 

 ized cells — the epidermis — sometimes provided with stomata. 

 Other portions of the plant, aside from the sporangia, are- 

 destitute of a true epidermis or of stomata. 



121. — The epidermal systems of Pteridophytes and Phaner- 

 ogams are so much alike that they may be described together, 

 although it must be remembered that in the lattei- group- 

 they are, in general, somewhat more perfect than in the for- 

 mer. In these groups the epidermal 

 structures consist usually of three por- 

 tions : (1) a layer of more or less^ 

 modified parenchyma — the epidermis 

 proper — bearing two other kinds of 

 structures which develop from it, viz., 

 (2) trichomes, and (3) stomata. 



122.— Epidermis. The differentia- 

 tion of parenchyma in the formation 

 of ejiidermis, when carried to its ut- 

 most extent, involves three different 

 modifications of the cells, viz., (1) 

 stomata oFSctotopo™ change of form, (3) thickening of the 

 r& cdis^^^^oidtrw'o: ^^alls, (3) disappearance of the proto- 

 op'l'JSg,""^UuVi'arge" pl'^^"^''' Contents. These three modi- 

 «f guard-cells —Aftei- Sachs, fications may occur in varying de- 

 grees of intensity ; they may all be slight, as in many aquatic- 

 plants and in the young roots of ordinary plants ; or the cells 

 may change their form, while there may be little thickening 

 of their walls, as m other aquatic plants, and some land plants- 

 which live in damp and shady places ; or on the other hand, 

 the change of form of the cells may be but little, while 

 their walls may have greatly thickened, resulting in a disap- 

 pearance of their protoplasm, as may be seen in parts of 

 some land plants which grow slowly and uniformly. When 

 the differentiation of epidermis is considerable, it can usu- 

 ally be readily removed as a thin transparent sheet of color- 

 less cells. 



