CHAPTER XL 

 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 independent 

 plants. But it is here requisite briefly to note the modifica- 

 tions undergone by them where they become components 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 the surface of 

 a leaf, a single example. In this it 

 will be seen that the epidermis cells 

 c, covered by the secreted external 

 layer a, and separated from the layer 

 of cells below them by the masses of 

 inter- cellular substance b, have differ- 

 entiations of form clearly related to ^•^'^ 

 differences in the incidence of forces. Their djTcrgences from 

 primordial sphericity are such as correspond with the un- 

 likenesses in the circumstances of their respective sides. 

 Similarly with the layers below them. And throughout the 

 more complex modifications which the cells of other tissues 

 exhibit, the like correspondence holds. 



Among plants of a lower order of aggregation, we hare al- 

 ready seen how cells become metamorphosed as they becrane 

 fntegrated into masses having definite organizations. The 

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