How The Plant Feeds From The Soil
Plants receive their nourishment from two sources--from the air and from
the soil. The soil food, or mineral food, dissolved in water, must reach
the plant through the root-hairs with which all plants are provided in
great numbers. Each of these hairs may be compared to a finger reaching
among the particles of earth for food and water. If we examine the
root-hairs ever so closely, we find no holes, or openings, in them. It
is evident, then, that no solid particles can enter the root-hairs, but
that all food must pass into the root in solution.
An experiment just here will help us to understand how a root feeds.
Secure a narrow glass tube like the one in Fig. 22. If you cannot
get a tube, a narrow, straight lamp-chimney will, with a little
care, do nearly as well. From a bladder made soft by soaking, cut a
piece large enough to cover the end of the tube or chimney and to
hang over a little all around. Make the piece of bladder secure to
the end of the tube by wrapping tightly with a waxed thread, as at
B. Partly fill the tube with molasses (or it may be easier in case
you use a narrow tube to fill it before attaching the bladder). Put
the tube into a jar or bottle of water so placed that the level of
the molasses inside and the water outside will be the same. Fasten
the tube in this position and observe it frequently for three or
four hours. At the end of the time you should find that the
molasses in the tube has risen above the level of the liquid
outside. It may even overflow at the top. If you use the
lamp-chimney the rise will not be so clearly seen, since a greater
volume is required to fill the space in the chimney. This increase
in the contents of the tube is due to the entrance of water from
the outside. The water has passed through the thin bladder, or
membrane, and has come to occupy space in the tube. There is also a
passage the other way, but the molasses can pass through the
bladder membrane so slowly that the passage is scarcely noticeable.
There are no holes, or openings, in the membrane, but still there
is a free passage of liquids in both directions, although the more
heavily laden solution must move more slowly.
A root-hair acts in much the same way as the tube in our experiment,
with the exception that it is so made as to allow certain substances to
pass in only one direction, that is, toward the inside. The outside of
the root-hair is bathed in solutions rich in nourishment. The
nourishment passes from the outside to the inside through the delicate
membrane of the root-hair. Thus does food enter the plant-root. From the
root-hairs, foods are carried to the inside of the root.
From this you can see how important it is for a plant to have fine,
loose soil for its root-hairs; also how necessary is the water in the
soil, since the food can be used only when it is dissolved in water.
This passage of liquids from one side of a membrane to another is called
_osmosis_. It has many uses in the plant kingdom. We say a root takes
nourishment by osmosis.