Fig. 112.
Germinating pea placed in
a horizontal position.
Fig. 113.
In 24 hours gravity has caused
the root to turn downward.
Figs. 112, 113.—Progeotropism of the pea root.
The stem, on the other hand, which was placed in a horizontal position has become again erect. This turning of the stem in the upward direction takes place in the dark as well as in the light, as we can see if we start the experiment at nightfall, or place the plant in the dark. This upward growth of the stem is also influenced by the earth, and therefore is a case of geotropism. The special designation in the case of upright stems is negative geotropism, or apogeotropism, or the stems are said to be apogeotropic. If we place a rapidly growing potted plant in a horizontal position by laying the pot on its side, the ends of the shoots will soon turn upward again when placed in a horizontal position. Young bean plants growing in a pot began within two hours to turn the ends of the shoots upward.
Fig. 114.
Pumpkin seedling showing apogeotropism.
Seedling at the left placed horizontally,
in 24 hours the stem has become erect.
Horizontal leaves and shoots can be shown to be subject to the same influence, and are therefore diageotropic.
261. Influence of light.—Not only is light a very important factor for plants during photosynthesis, it exerts great influence on plant growth and movement.
