23 



C'HAPTEE III. 



CELLS OF THE EXTERNAL TISSUES AND CERTAIN 

 SUPPORTING AND PROTECTIVE TISSUES IN PLANTS. 



A. C'EIiS ARISING FROM THE DERMATOGEN. 



1. The Epidermis and Structures in connection with it. 



The epidermis forms the outermost layer of cells occurring in 

 such of the higher, and also lower, plants as possess differentiated 

 organs ; the layer forms, as a rule, a protective covering to the 

 more delicate tissues beneath, and, moreover, is intimatelj" con- 

 cerned in the function of transpiration and the admission and 

 means of exit of the gases of respiration and assimilation, 

 matters which will be examined more fully when the stomata 

 are studied. 



Fig. 7. — The Dermatogen from the Apex of a Bud. — The outer layer 

 of young cells represents the derniatogen, the deeper cells belonging 

 to the "periblem." 



Every epidermal cell is at first, like all young cells, a thin- 

 walled undifferentiated structure; the developing epidermis is 

 best examined in thin longitudinal sections taken through the 

 apex of a young shoot of Ahies or Pinus. In such a section the 

 following features may be noted (see Figs. 7 and 34) : — 



(a) An outer layer of small cubical cells, filled with protoplasm, and 

 possessing relatively large nuclei. 



