seedlings. They are best seen in the seedlings grown on damp sponge. In
those grown in sand, they become so firmly united to the particles of
soil, that they cannot be separated, without tearing the hairs away from
the plant. This will suggest the reason why plants suffer so much from
careless transplanting.

The root-hairs have the power of dissolving mineral matters in the soil
by the action of an acid which they give out. They then absorb these
solutions for the nourishment of the plant. The acid given out was first
thought to be carbonic acid, but now it is supposed by some experimenters
to be acetic acid, by others to vary according to the plant and the time.
The action can be shown by the following experiment, suggested by Sachs.

[Illustration: Fig. 12. I. Seedling of _Sinapis alba_ showing root-hairs.
II. Same, showing how fine particles of sand cling to the root-hairs.
(Sachs.)]

Cover a piece of polished marble with moist sawdust, and plant some seeds
upon it. 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