THE BOTANY OF THE APPLE TREE. 21 



particular conditions of the soil. A soil may be wanting in certain 

 substances necessary for the food of the tree: the result will be the 

 starvation of the tree. But a soil may be fertile and still not supply 

 enough food. If so hard that the roots cannot penetrate it, the tree 

 is starved as certainly as in a barren soil. Or if the soil is too dry 

 starvation will result, here intensified by the lack of water, the most 

 important of the food substances. But on the other hand, in a 

 soil which is too wet the tree is starved again, because its roots are in- 

 jured by submersion, while the coldness of such a soil also acts dele- 

 teriously, both resulting in an under supply of food. The presence of 

 excessive amounts of potash (and some other salts) in the soil may de- 

 stroy the young roots, resulting again in starvation. 



II. UNFAVORABLE ATMOSPHERIC CONDITIONS. When the air is 

 very dry the loss of water from the leaves is excessive and may exceed 

 the supply obtained by the roots from the soil. Upon the plains the 

 prolonged periods of high dry winds are peculiarly trying to all vege- 

 tation, and especially so to such trees as the apple, which has not only 

 to meet the water-loss from its leaves, but also that from its flowers, 

 and afterwards from its crop of fruits which remain on the trees during 

 the summer. No doubt our trees are much more enfeebled from this 

 oause than we are wont to believe. We know that a greenhouse plant 

 which is kept for a long time in a semi-wilted condition is eventually 

 much injured, and there can be no doubt that the same is true of ap- 

 ple trees. 



It is possible that occasionally the temperature of some parts of the 

 tree may rise too high, but upon this point we still lack sufficient data. 

 I have no doubt, however, that in hot summer days the exposed trunks 

 reach a dangerously high temperature. 



The reverse condition of extremely low temperature, which is 

 reached in many winters, is equally dangerous. A healthy apple tree 

 will endure 20 or more degrees below zero of Fahrenheit, and some 

 varieties will survive 40 or even 50 degrees below zero. Yet the ex- 

 perience of many Iowa apple growers twelve or fourteen years ago, 

 who lost in one disastrous winter all. the trees they had successfully 

 grown for twenty years, must be a warning to us. 



A quick alternation between a low and a high temperature is usu- 

 ally fatal to parts of living plants. This is probably the cause of 

 " sun scald.' 7 When the frozen tree is suddenly thawed out by the 



