22 



FOUNDATIONS OF BIOLOGY 



the same type of unit. And if we extend our study to other 

 parts of the Buttercup or the Frog or, indeed, to any part of 



FIG. 5. Vertical section (highly magnified) of a leaf to show its cellu- 

 lar structure, a, guard cells, at opening (stoma) through epidermis; 

 b, cells containing chlorophyll; c, upper and lower epidermal cells. 

 (From Abbott, after Bailey.) 



any familiar plant or animal, we find essentially similar units 

 of structure in every case. In fact, the bodies of all living 



FIG. 6. Transverse section (highly magnified) of a simple animal 

 (Hydra) to show the cellular structure. Outer layer, ectoderm; inner 

 layer, endoderm; central cavity, enteron. (After Shipley and 

 McBride.) 



things either consist of a single organic unit or are congeries 

 of millions of essentially similar units called CELLS. (Fig. 4.) 



