CHAPTER XI. 



THE SHAPES OF VEGETAL CELLS. 



237. WE come now to aggregates of the lowest order. 

 Already something has been said ( 217) concerning the 

 forms of those morphological units which exist as indepen- 

 dent plants. But it is here requisite briefly to note the 

 modifications undergone by them where they become compo- 

 nents of larger plants. 



Of the numerous cell-forms which are found in the tissues 

 of the higher plants, it will suffice to give, in Fig. 254, re- 

 presenting a section of 

 a leaf, a single example. a 

 In this it will be seen 

 that the cells forming 

 the upper and lower sur- e 

 faces, a and &, have dif- 

 ferences of shape related d 

 to differences in the inci- j 

 dence of forces: they are 

 more or less flattened in 



relation to the environment. The underneath cells at c, 

 form a class which, similarly exposed to light at their 

 outer ends, and, as we may assume, largely developed in 

 adjustment to their active assimilative functions, are, by 

 mutual pressure, made to grow more in the direction of their 

 lengths than in the direction of their breadths. Then on 

 the other side we see that the cells d, next above the outer 

 layer, while approximately similar, become more and more 

 dissimilar as they diverge from the surface, ard are quite 



175 



