You can see how very determined the directions of growth are by planting upside down a bean which is just beginning to sprout, so that its root points up into the air. As it grows you will see the root bending over till it points vertically downwards, while the stem bends up and grows straight into the air (see fig. 29). The same thing happens if you plant a seedling on its side, and even if you take quite a big seedling, which has grown in the usual way, and then place it upside down in moist air, you will see the root and shoot bending in order to get into their right positions. This very determined growth on the part of roots and stems seems to show us that they must have some means of “perceiving” and regulating their position. It is not an accident that they always grow in these very definite directions. Let us find out what we can about this question.
Take a seedling and mark its root as you marked the roots for the experiment on the region of growth (see fig. 28), lay this seedling on its side on soft, damp sawdust, so that the root can easily bend into it. Next day you should find that the end of the root has bent, and that the bend is in just about the same region as that which showed the most active growth.
Is this actively growing and bending region therefore the part of the root which “realizes” that the whole is in a wrong position, and which therefore bends to put it right?
To answer this question quite fully would require a great deal of work, but there are three simple experiments which you can do, and which will tell you the most important facts about it.
(1)[5] Take a seedling with a fairly long root which has been growing straight down, then very quickly and with a sharp knife or razor, cut off the last 2 mm. of the tip of the root. Lay the seedling on its side on damp sawdust and examine it next day. It will not have bent, even though it has grown in length (see fig. 30, A).
Fig. 30. Experiments on the bending of the root tips in Beans. (See description in text.)
(2) Take another like it and leave it lying on its side for an hour, and then cut off the tip in the same way as in number one, placing it on its side once more. Next day you will find that it has bent in the same way as one which had not been cut (see fig. 30, B).
(3) Take a third, as like the other two as possible, and lay it on its side all night; do not cut it till next day, when it has definitely begun to bend (see fig. 30, C), then quickly cut off the tip, and place it in the upright position (C1). You will find that it continues to grow in the bent form, the root tip going on to one side. It does not seem to know that it is growing along instead of down. If you keep it in this position for a few days it will then get a new tip and begin to grow downwards in the usual way (see fig. 30, D).