We see in this way that the roots take in a large and continual supply of water, and this must get pressed up the stem even without the influence of the transpiring leaves. This is called the “root pressure,” and is a very important factor in supplying the plant with water. In a plant which is growing under usual conditions, both the transpiration of the leaves and the root pressure are at work, and are both necessary to keep a good stream of water passing through the plant. This stream of water provides it with its mineral food materials, and also keeps it stiff and fresh, and is, as we have seen, absolutely necessary for the growth of the plant.
CHAPTER VIII.
LIGHT AND ITS INFLUENCES
When we were experimenting on the building of starchy food in leaves (Chapter VI.) we saw how very important and even essential light is for the activity of the plant, and it is therefore natural to expect that light should influence its growth very considerably.
You may see the effect of light which comes only from one side on plants grown in the windows of rooms. If they are left in one position they grow in a one-sided manner with only the bare stalks toward the darker side of the room and all their leaves turned towards the window through which the light comes. If you want them to look pretty towards the room side also, they must be turned round frequently, so that the leaves are drawn in many directions instead of one only. The usual effect of light is to make the leaves grow towards it. You may see this still more clearly by placing a pot of seedlings in a blackened box with a small hole on one side. Very soon they will bend over towards the light entering by it (see fig. 23).
Fig. 23. Grass seedlings growing in an earthenware dish enclosed in a strong box blacked inside so that the light only enters at a. (Note how the seedlings bend towards it.)
Leaves can absorb most light when their upper surfaces are at right angles to it, and you will find some leaf-stems will bend right round in order to allow their leaves to get into this position. For example, if you take a pot of nasturtiums growing in the usual way, and support the pot on a stand, and cover it over with a bell jar which has been blackened, or with a black box, so that all the light reaches the plant from below, you will find that in a day or two the leaves will have turned completely round on their stalks and are now facing the light, so that they are upside down in their relation to the position of the whole plant (see fig. 24).