152 TRANSACTIONS OF THE ILLINOIS 



Many illustrations of the above points might be given, but one 

 or two will suffice. I have in my yard a peaish tree that, during the 

 summer of 1884, hung so full of fruit that several of the limbs were 

 broken down, and afterwards these were removed from the tree. 

 One of these Avas a large limb on the southwest side, and another on 

 the south side of the tree. Up to this time the trunk was sound 

 and healthy, and well protected by the limbs. The next season a 

 large patch of blackened bark was to be seen on the southwest side, 

 the result of the thawing and freezing during the winter of 1884-5, 

 and the subsequent souring of the sap during the following summer. 

 During the past season the whole of that side of the tree has died, 

 involving one limb as much as three years old, that was alive last 

 year. Another tree in the same yard, that leans to the northeast, has 

 the whole southwest side dead and devoid of bark. If we want illus- 

 trations from the forest trees, I may say that nearly every day I pass 

 two trees, one a black locust and the other a catalpa, both leaning to 

 the northeast, that are good examples. In each case the southwest 

 side is dead from the ground to the limbs. In neither case has any- 

 thing else seemed to interfere with the growth, and gradually the 

 new growth is extending along the edges of these large places, the 

 bark, as the tree becomes larger, becoming rougher, and in a measure 

 acting as a protection. 



It ma}^ be asked how a tree with low, spreading branches, which 

 if covered with leaves, would shelter the trunk from the sun in sum- 

 mer, can protect the same trunk when the tree is leafless? During 

 winter the sun shines through these branches upon the large limbs 

 and trunk in such a way that part of these will be in sunshine and 

 part in shadow of the branches. As the sun apparently passes from 

 east to west, these shadows move across the tree in an opposite direc- 

 tion, so that no part of the trunk is long under the sun's influence, 

 the cooling while in the shadow counteracting the heating while in 

 the sunshine. The influence of these moving shadows is often seen 

 in a hot-bed that has become so heated as to kill the plants. Under 

 the centres of the sashes, where the sun shines with unabated ardor, 

 there may be no plants left, but a strip two or three times the width 

 of the timbers, upon which the sashes rest, will be uninjured. If the 

 sun's rays were all the time the same on all the parts, there should 

 be a space just the width of these timbers uninjured, but the moving 

 shadows have protected strips as far as they reached. 



This question of shadows, moving or not, may be used here to 

 explain why some forest trees are affected by sun-blight, while others 

 are not. We may say, as a general rule, if a tree has thin, smooth 

 bark, it will be subject to this disease if the trunk be exposed, while 

 trees with rough, thick bark are in a measure exempt. The rough- 

 ness of the bark, added to its thickness, prevents the tree from be- 

 coming warm enough to sufficiently start the flow of sap to cause a 

 disintegration of the contents of the cells by freezing, and conse- 



