33 
The Plant 
brown marks on the marble where the roots have been 
lying. We may even wash it, but the marks remain. 
Therefore we know it is not only soil, but is in reality 
a change which has taken place in the marble itself, 
brought about by the action of weak acid which was given 
out by the root hairs. This acid has dissolved the car- 
bonate of lime contained in the marble, and some has been 
absorbed by the plant as its food. 
Now, roots are living things, and require air as well as 
moisture. Therefore we must keep our soil in a sufficiently 
open condition to prevent the water from accumulating, 
and thus keeping out the air that should have access to 
the roots. If such a state arises it must be remedied at 
once. We must, of course, always have sufficient moisture 
in the soil, as this not only dissolves the food substances, 
but also acts as their carrier right through the plant to the 
leaves, and even back again from the leaves to the growing 
points of roots, shoots, leaves, fruit, &c. If there should 
be a scarcity of moisture at any time, the little tubes of 
which the roots and stems are composed get filled with 
air, then no more moisture with food substances can pass 
along them, and the plant will die. Therefore, though we 
should never have an excess of moisture, yet we must on 
no account allow our plants to get too dry. Sometimes, 
when the soil appears to be dry, there may be sufficient 
moisture in it for the needs of the plant. A little expe- 
rience, with careful observation, will soon enable us to 
understand when plants require our aid in providing them 
with moisture. 
Let us now examine the stem, and we shall at once see 
that it consists of many parts. First, there is the main 
( C 525 ) 4 
