Page:Popular Science Monthly Volume 46.djvu/413

Rh of the leaf ceases at the moment when all the water brought up is claimed by these leaves. Correlative operations then come into play. The leaf parts in process of formation then assume the shape of bud scales, and all further advance in growth is interrupted. Very simple examination will show that this conclusion of the terminal bud is not caused by any deficiency of food. If the growing shoot is brought into a room saturated with moisture, the bud is not formed. Evaporation is arrested in such a room; there is no want of water; and therefore the mechanism is not unloosed which restricts the further development of the bud. Buds already closed may also be started in a new growth in a room saturated with moisture.

The closing of the buds in the axils of the leaves is conditioned on similar causes. These buds may be easily made to expand if the branch is cut off or stripped of leaves. The advantage to the plant of this property of restoring lost leaves is seen after every instance of devastation by caterpillars. If we cut off a young potato stem just above the ground, the parts growing beneath will at once come up out of the earth. They are the threadlike shoots which would otherwise swell at the ends and form potatoes. The want of incoming nourishment acts as a stimulant to the subterranean growth, and excites processes in them by which their nature is completely changed. Instead of, as before, growing straight down into the soil, the sprout directs its end upward, soon appears above the surface, produces green leaves instead of scales, and puts on the appearance of the ordinary leafy shoot. Corresponding in principle with the behavior of this potato plant is that of trees which give their side limbs an upward direction when they have lost their leader.

The internal and external changes which a fir limb has to undergo in order to become a leader are hardly less far-reaching than the transformation of an underground potato shoot into an aerial foliage stem. We perceive that the change of the course of the water and food stuff to the highest limb, which occurs after the removal of the leader, is the correlation force by which the metamorphosis is brought about; for these causes operate as stimuli to produce a change in the relations of light and gravity, by which a doubly differentiated organ is converted into one reacting on all sides alike—in a word, the whole living mechanism is profoundly transformed. When the metamorphosis of the lateral limb into a leader is completed, it can no longer be diverted from its upright position, but returns to it with the same persistency with which it formerly resumed the horizontal position.

Not less striking are other phenomena of correlation connected with the processes of transpiration, but which can be studied only under the microscope. The moisture which plants give out to the