VOL. III.] Forms of Trees. 7 



vent the sap from circulating. In course of time injurious insects, 

 such as borers of various kinds, find their way through crevices, and 

 parasites gradually destroy the trees. Trees which are thus es- 

 pecially tender are, among cultivated trees, apples and pears, and 

 among wild trees, weeping willows, poplars, young oaks, maples, 

 etc. A tree when once injured seldom recovers if left to itself, but 

 dies or at least becomes sickly. In order to counteract this fatal 

 force of excessive light and heat combined, the horticulturist en- 

 courages lower limbs and foliage, prunes his trees low, or otherwise 

 shades the exposed parts. Nature works very much in the same 

 way. Young trees growing in heated regions are covered with 

 lower limbs thickly set with foliage, or develop large weeping tops 

 or crowns with drooping branches, which shelter the tender stems 

 as effectually as if they were covered with an umbrella. That such 

 a shade is absolutely necessary can be clearly demonstrated. There 

 is, for instance, no more tender tree than our common weeping 

 willow, a native of the hot region of Asia Minor. This tree flour- 

 ishes even in our warmest regions under proper conditions of 

 moisture, as long as its natural form is not interfered with. But let 

 anyone prune back its limbs and cause the direct rays of the hot 

 sun to strike its trunk, and the tree will soon become diseased 

 and die. The dying of weeping willows is common all over the 

 warmer parts of this state, and is everywhere to be principally 

 ascribed to the cutting away of limbs and to the entrance of heat 

 and direct light. 



The excessive heat and light has also a bad eftect upon the ground 

 in places where rain or other moisture is scarce. The sun dries out 

 the soil and makes it too dry for the trees and plants. To counter- 

 act this heat, nature causes lower limbs to spread out as close to the 

 ground as possible, or furnishes the tree with large dense and 

 rounded crowns which cover the soil with shade and prevent the 

 moisture in the immediate vicinity of the trunk and roots from dry- 

 ing out. 



Nature furnishes also other remedies, such as peculiar position 

 of the leaves, tough and hardy bark, gray and light colors of 

 leaves and stems, hairs or cells especially constructed to withstand 

 evaporation or heat. 



While the snow especially affects evergreens, the heat and light 

 affect evergreens and deciduous trees almost alike. 



