93 BOTANY. 



horizontal division the several superimposed circular rows 

 of cells are formed. 



120. In true mosses the sporangia possess an epidermal 

 system which 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 latter 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 epidermis, when carried to its ut- 

 most extent, involves three different 

 modifications of the cells, viz., (1) 



Fig. 79. Top view of two > < /n\ >i i \ 



stomata of Marchantm poly- change of form, (2) thickening of the 

 2?$Si SuTOid^ walls, (3) disappearance of the proto- 

 renlng^truch^rge?; P lasmic contents. These three modi- 

 si, guard-ceils After 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 in 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. 



