teil, 
THE LEAF 39 
leaf the photosynthetic cells are situated around the parallel 
veins (Fig. 31), an arrangement which facilitates the removal 
of food products. 
HYDRATION 
We have seen that a large part of the stiffness of leaves is 
due to water contained in their cells. The forces that hold 
water within the cell walls are not in the walls themselves but 
in the cell contents. The cell walls are very permeable to water; 
that is, water can pass through them very readily. If there were 
no forces in the cell contents to hold the water, the water would 
leak out through the walls. The term hydration may be used 
to denote the absorption and holding of water, without regard 
to the manner in which this is accomplished. The hydration of 
the cell contents is, then, the absorption and holding of water 
by the cell contents. This is due to two forces: the hydration 
of colloids in the cell and osmotic pressure. 
Hydration of colloids. In the discussion of the colloidal nature 
of protoplasm, in the chapter on the cell, it was pointed out 
that many colloids are capable of hydration (that is, of absorb- 
ing and holding water) and that protoplasm contains hydrated 
colloidal particles. Protoplasm is capable of absorbing and re- 
taining large quantities of water. When protoplasm absorbs 
water, it tends to swell and stretch the cell wall and thus give 
rigidity to the cell. 
Osmotic pressure. When a substance is in solution, the dis- 
solved substance tends to become equally distributed or diffused 
throughout the liquid (solvent) in which it is dissolved. This 
can be illustrated by the following example: If a small quan- 
tity of sugar is placed at the bottom of a vessel containing 
water, the sugar will go into solution and, even if there is no 
movement of the water, will become diffused throughout the 
water so that all of the water will have a sweet taste. Dzffusion 
is very important in plant physiology, for by this means sub- 
stances in solution in one part of a plant are frequently carried 
to other parts. 
