70 
SECOND BOOK. 
food and water ; but the walls of their cells will not 
let any solid pieces, however small, pass them. So, 
whatever food goes in must first be dissolved, just 
as you have seen sugar dissolve; then it can pass 
through with the water that holds it. If the water 
cannot itself dissolve the plant-food, an acid sent 
out by the little root-hairs does so. 
7 . But the plant gets a very small share of its 
solid substance from the soil. If we thoroughly 
dry a plant, to get rid of its water, and then burn 
the solid part, most of it will pass away and we 
shall have left nothing but a small quantity of ash. 
That ash is almost all that the plant took from the 
soil, except water. 
8. Since the plant takes up so much water from 
the soil, we might suppose that its cells would at 
last become so full and swollen that no more could 
be taken in. But if that happened the water would 
soon cease to pass along from cell to cell, and the 
plant would die. So it has a way of getting rid of 
some of its water to make room for more to come 
in. 
9. The water escapes through the surface of its 
leaves. It goes away as vapour, which we cannot 
see, just as the water of a pool in drying up passes 
unseen into the air. Then, as the water escapes 
from the leaves, more rises through the roots to 
take its place, and in that way a constant stream is 
kept up through the plant. If the roots cannot 
