19.6 THE POPULAR SCIENCE MONTHLY. 



that change of temperature alone is deleterious to vegetable growth ; 

 and we may, perhaps, justly infer that uniformity of temperature, 

 when it lies at or near that degree which is most favorable to healthy 

 vegetable activity, is a desirable point in a fruit-growing climate for 

 the even, early, and better maturing of fruit. 1 



The influence of forests on rainfall is still an unsettled question. 

 It is a very general impression that forests, in some way, promote the 

 fall of rain when it would not occur if the same region was bare of 

 trees. A great array of authorities may be quoted in favor of this 

 view. It is believed that Spain, parts of France, Switzerland, and the 

 Tyrol, Northern Africa, Persia and Palestine, Egypt and India, the 

 islands Malta and Mauritius, the Cape Verd Islands, and most of the 

 West Indies, have either been turned into deserts or greatly injured 

 by the destruction of their forests, and the blight and droughts which 

 have followed. It is alleged, too, that the planting of forests has in 

 some instances, as in Scotland, Egypt, and St. Helena, caused a more 

 abundant rainfall. But these alleged results, though supported by 

 great names in science, are disputed. Forests may affect rainfall for 

 any thing positively known, but the evidence that they do so is not 

 such as science can accept." But, however this may be, they have 

 doubtless much to do with the benefit which vegetation receives from 

 the rain that does fall. 



In a country quite destitute of timber the surface would dry off 

 much more quickly, in consequence of the free sweep afforded to the 

 winds. The water from rains would also pass over the surface more 

 freely into the brooks, and be thus lost to vegetation. The spongy 

 surface of the forest absorbs a larger proportion of rain than the open 

 fields, and thus retains it in the soil as a source from which the neigh- 



1 Both De Vries and Sachs ascertained that every kind of plant has its special degree 

 of temperature at which it makes most growth in a given time ; but, while Koppen rec- 

 ognized this, his investigations have made an addition to our knowledge of the subject, 

 hi3 point being that the plant grows more when kgpt at a uniform temperature than if it 

 had varied between extremes of which this temperature is the mean, thus showing that 

 variation of temperature acts as a check on growth. 



According to Karsten, great and sudden changes injure the health and hardiness of 

 plants; while De Vries comes to a directly opposite conclusion. This, however, does 

 not affect Koppen's result, which has reference to rapidity of growth. Moreover, even if 

 great daily variation of temperature should not affect the health of plants, it might, 

 nevertheless, be not wholly harmless to the tenderer fruits. 



The preceding paragraphs have been suggested by the kinship between forests and 

 lakes in their influence on climate and fruit-growing. 



8 Since writing the above, we have happened to fall upon several statements in favor 

 of the influence of forests on rainfall, some of them from respectable scientific sources, 

 Proctor, Bryant, Colver, etc. I learn, however, that Prof. Henry, of the Smithsonian 

 Institution, has recently reported that observations for the last twenty years in this coun- 

 try show no appreciable influence of forests on the amount of rainfall. This should 

 carry much more weight with it than the mere fashion of opinion about forests causing 

 rain. 



