136 



which require Hght do not show this difference. The assimilation activity 

 of the flora of the forest floor is very rapid in spring when the trees and 

 trunks are still bare and decreases with the foliation more slowly in shade 

 trees, because of their structure, until it finally ceases entirely. The respira- 

 tory intensity decreases with the decreased "food consumption." Detached 

 shade leaves of Convallaria majalis etc., form more starch in the sun as well 

 as in the shade than do sun leaves treated in exactly the same way and they 

 also fix carbon dioxid more rapidly in the same amount of light than do 

 these. Moreover in Convallaria the storage of starch was found to be less, 

 the drier the soil. Equally large leaf surfaces containing palisade cells tran- 

 spire much more strongly than do those leaves having the structure of shade 

 leaves. 



It is evident from these statements that changes of great importance 

 must take place in the economy of trees accustomed to shade, when suddenly 

 exposed to light, viz., when left standing by removing parts of forests. In 

 parks too strong and sudden an exposure to light by the removal of num- 

 erous trees not infrequently results in the partial or total death of the 

 crowns of the specimens left standing. 



We must turn our attention to still another point. If plantations of 

 fruit trees along streets on level land, especially cherries, be examined, many 

 cases will be found wih trunks split open on the south or southwest side, 

 with the bark torn into tatters and often showing lumps of gum on the 

 wounded surfaces. These injuries are ver}' evidently due to frost. The ex- 

 planation lies in the fact that the level, cleared lands are exposed in spring 

 to extreme temperatures. The February and March sun shining intensely 

 on the trunks, and strengthened in its action by the reflection from the soil, 

 starts the reserve plant food prematurely and the tissues, being richer in 

 water and sugar, at once succumb to the action of the frost. A moister 

 atmosphere in the neighborhood of water or wooded areas equalizes the 

 temperature and senses as a protection from frost. 



Naturally in regions with greater soil elevations and more noticeable 

 dift'erences between valleys and mountains these factors co-operate determi- 

 natively and often decisively, but on the plains the forestration is a very 

 considerable factor. Cutting considerable forest tracts on wide plains often 

 is a source of injury avenged not only on the owner but on the whole neigh- 

 borhood, since it increases the chance for damage from late frosts. In this 

 connection many small forest tracts, scattered over a large plain, would be 

 of use since no considerable distant action from one single large forest may 

 be reckoned upon. There is a further advantage to be derived from 

 forests, — that of protection against the wind (windbreak) when there are 

 no mountains. 



Just as every bright side has its shadow, forests can exert an injurious 

 influence on the adjacent fields. The forest properly located can withhold 

 the summer rains, usually coming from the west, from a given field so that 

 there will be dry, windless streaks across the fields in its immediate prox- 



