Page:Popular Science Monthly Volume 46.djvu/412

398 much. or even more work on the smaller leaf as the dimmer light of the shady situation on the larger one. An equivalent gain is realized by the lengthening of all the parts of the branch which takes place in the obscure situation. The leaves are thereby farther separated on the limbs, and do not cover one another. It is different in the isolated tree, where the leaves, even in a compressed situation, do not suffer for want of light. Hence, too, the great difference in the appearance of the conical crown of a solitary beech with its thickly compressed limbs and leaves, and that of the loosely spread out, umbrella-shaped undergrowth which the red beeches form in the shade of the wood.

A dry location promotes the same processes of growth as increased transpiration. The need of a plant in a dry soil is to reduce transpiration, and correlative processes are manifested through which that result is reached. The growing plant is so affected as to acquire a similar structure to that of a plant in a very sunny situation. The correlative operations in many plants take the form of giving a hairy covering to their leaves. A layer of air-retaining hairs diminishes evaporation. Hence the same plant may be hairless in moist ground, and in dry be covered with numerous hairs. The same is the case in plants from which the water supply is taken away. Superfluous growths are produced on the leaves and moderate transpiration. A slimy content is developed in the leaf cells of many plants, and serves to retain the moisture that is present within them. A dry location also generally promotes a greater thickening of the cells on the leaf surfaces, by means of which evaporation is made more difficult and prolonged. In the very cold, long-frozen soil of the arctic tundras plants have difficulty in obtaining the water they require; and in such situations, notwithstanding the real abundance of water, the same correlations in structure are found a*s in dry soil. The leaves are small and thick, and form slime within, and thicker cell walls on their surface. Similar rules obtain with plants in the saline soils of the steppes and the seashore Diversified conditions thus co-operate to produce the most favorable aggregate of life conditions for the plant.

The processes of transpiration in the plant appear adapted to introduce us into the difficult field of correlation, from the fact that it affords us an easier way than we can find in many other cases of looking into their mechanism. All the numerous instruments which perform the work in the organism interlock, mutually affecting and conditioning one another.

The researches of Julius Wiesner have shown that transpiration is the principal element in controlling the termination of the end bud. The vessels of our woody plants convey only a limited quantity of water to the unfolding leaves in spring. The growth