The force that makes a tree grow erect must be strong enough to overcome the force of gravitation. We are likely to forget that every moment the latter force is trying to pull trees to the ground. Careful observation will see the effects of the struggle between the two con tending fumes. An excellent illustration is seen in the gradual bend ing of old branches away from the trunk. This is supposed to be due to their weight, and to accompany the loss of vigor in the tree. Young oaks have acute angles between branch and stem. Observations show these same branches grown old to stand horizontal to the trunk and sometimes to droop. Horse chestnut trees have ascending branches when young. An old tree shows the branches curving first out, then down, then up. supporting the last few years of growth and the ter minal leaves in an upright position.
The upper limbs of spruce and pine trees are lifted up. Lower down the branches are horizontal. The oldest, heaviest ones. droop in decrepit attitudes, and often lie passive upon the .he mond.
The only ones of our common trees that do not pass through these modifications of shape and position of limbs are the fastigiate trees, those of the Lombardy poplar type.
It will be observed that the branches of these trees never grow large nor long. There is another influence besides gravitation which acts against the tree's aspiring tendencies. It is the wind. A careful observer has only to look at the trees of a region to learn the direction and strength of prevailing winds. It is a fascinat
ing study from car windows, relieving many a tedious journey. 1t is thin solitary trees which aro chiefly affected. Trees in groves or forests defend each other against the winds. The reader will do well to pause here and look at the full - page plates of White Pine, Tamarack, Silver-leaved P oplar, and American Beech. They tell some interesting stories about prevailing winds. Compare the Horse Chestnut, and the White Oak, and others. These trees have grown in protected situations.
In the northern woods the forester's compass is the tree top — the soft tapering terminal shoot of hemlock and other conifers bent over by the winds. There rare hundreds of them always in sight. In regions where lie is acquainted he needs no better guide-posts than these. They are not all alike, and so they chart the forest for him, as familiar objects guide us on our way through the city.
tree grow erect? In plain English, it is the crapiny fin. liyht. LI.uwever, if you wish technical terms you may use heliotropism " for this ten dency to seek the light, and " geotropism " fur the tendency of plants to obey the force of gravitation. It is the desire fur light that makes trees grow tall in the forest. It is the struggle for light that makes branches lengthen, that gives the leaves farthest away from the trunk the best chance to live and make a living for the tree,— that makes the inner recesses of the tree dark and leafless.