THE CELL. 



477 



laws regulating the structure of the organism, or of the tissue, of which 

 the cell forms part, e. g. unequal growth in particular portions ; and ob- 

 struction to the possibility of expansion in certain directions, from the 

 pressure of surrounding cells. 



These influences are very fruitful in producing variety of form. The 

 first kind is the most important, and determines the general form of the 

 cell ; the second in most cases affects merely the shape of its external 



Fig. 520. 



Fig. 521. 



Fig. 522. 



Fig. 520. Merenchymatous cells of the rind of Euphorbia canariensis. Magn. 100 diam. 



Fig. 521. Liber-cells of Cocos botryophora. Magn. 50 diara. 



Fig. 522. Parenchymatous cells from the leaf ot Orchis muscula. Magn. 200 diam. 



surface. The form of the cells of fully developed tissues is usually the 

 result of both kinds of influence combined. 



In cells existing in combination we find three principal classes of forms, 

 referable purely to the influence of the law of development : (1) the 

 spheroidal, obedient to the fundamental type ; (2) the cylindrical, in which 

 there is a more or less considerable tendency to elongate in the direction. 



