398 THE POPULAR SCIENCE MONTHLY. 



nmch. 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 there- 

 by farther separated on the limbs, and do not cover one another. 

 It is different in the isolated tree, where the leaves, even in a com- 

 pressed situation, do not suffer for want of light. Hence, too, the 

 great difference in the appearance of the conical crown of a soli- 

 tary 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 diSiculty in obtaining the water they 

 require ; and in such situations, notwithstanding the real abun- 

 dance 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 Diver- 

 sified 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 instru- 

 ments which perform the work in the organism interlock, mutu- 

 ally affecting and conditioning one another. 



The researches of Julius Wiesner have shown that transpira- 

 tion 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 



