CHAP. IT.] PLANT STRUCTURE AND GROWTH. 25 



of the cell is modified by contact at their ends, 

 changing them from the oval or round form of Pro- 

 tococcus into an oblong. In higher plants, we may 

 find them of every conceivable shape, but the modi- 

 fication is always due to this same cause pressure; 

 but we shall find it is exerted at the sides as well as 

 at the ends. 



Here in this diagram, at a, we have two cells 

 scarcely touching at their sides, so that they retain 



oo oo 



' I 



FIG. 24. 



their simple form ; but at b we suppose them to be 

 pressed together, and this pressure alters their form, 

 making flat the two sides that are pressed together. 

 At c we suppose that a number of cells are placed 

 side by side, and pressure exerted at both ends of 

 the series, consequently they assume an oblong 

 shape. At d they are pressed together from all 

 sides, just as people are pressed together in a crowd, 

 and the result is, the cells become many-sided (poly- 

 hedral), so as to occupy the least amount of space. 

 Now in different parts of a plant we have these cells 

 arranged and modified in different ways, and groups 

 of cells arranged in a definite manner are termed 

 tissues. The simplest form is cellular tissue, the cells 

 composing it retaining much of the normal form, 

 but frequently they become many-sided by the pres- 



