When the seedlings are somewhat grown, remove the sawdust, and
the rootlets will be found to have left their autographs behind. Wherever
the roots, with their root-hairs have crept, they have eaten into the
marble and left it corroded. The marks will become more distinct if the
marble is rubbed with a little vermilion.
In order that the processes of solution and absorption may take place, it
is necessary that free oxygen should be present. All living things must
have oxygen to breathe, and this gas is as needful for the germination of
seeds, and the action of roots and leaves, as it is for our maintenance of
life. It is hurtful for plants to be kept with too much water about their
roots, because this keeps out the air. This is the reason why house-plants
are injured if they are kept too wet.
A secondary office of root-hairs is to aid the roots of seedlings to enter
the ground, as we have before noticed.
The root-hairs are found only on the young parts of roots. As a root grows
older the root-hairs die, and it becomes of no further use for absorption.
But it is needed now for another purpose, as the support of the growing
plant. In trees, the old roots grow from year to year like stems, and
become large and strong. The extent of the roots corresponds in a general
way to that of the branches, and, as the absorbing parts are the young
rootlets, the rain that drops from the leafy roof falls just where it is
needed by the delicate fibrils in the earth below.
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