[Illustration: FIG. 65.
To show the giving off of gas by leaves, and that sunlight is
necessary for it. The jars contain seaweed. _A_ was set in the sun and
developed enough gas to float part of the plant. _B_ was left in the
darker part of the room and developed very little gas.]

[Illustration: FIG. 66.
Seedling radishes reaching for light.]

[Illustration: FIG. 67.
Elm leaves injured by the "imported elm-tree leaf beetle," a chewing
insect.]

The work of the leaves of house plants is often interfered with by not
giving them sufficient sunlight. Garden and field plants are
sometimes planted so thick that they crowd each other and shut the
light and air from each other, or weeds are allowed to grow and do the
same thing, the result being that the leaves cannot do good work and
the plant becomes weak and sickly. Weeds are destroyed by pulling them
up and exposing their roots to the sun. This should be done before the
weeds blossom, to prevent them from producing fresh seeds for a new
crop of weeds. Some weeds have fleshy roots--for example, dock,
thistle--in which food is stored; these roots go deep in the ground,
and when the upper part of the plant is cut or broken off the root
sends up new shoots to take the place of the old. Some have
underground stems in which food is stored for the same purpose. The
surest way to get rid of such weeds, in fact, of all weeds, is to
prevent their leaves from growing and making starch and digesting food
for them. This is accomplished by constantly cutting off the young