cHaP. 11.] 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 me 
Fic. 24. 
their simple form; but at 4 we suppose them to be 
pressed together, and this pressure alters their form, 
making flat the two sides that are pressed together, 
At ¢ 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 (foly- 
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 ce//ular tissue, the cells 
composing it retaining much of the normal form, 
but frequently they become many-sided by the pres- 
