Page:Popular Science Monthly Volume 2.djvu/208

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196
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.[2] 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 Köppen recognized this, his investigations have made an addition to our knowledge of the subject, his point being that the plant grows more when kept 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 Köppen'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.
  2. 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 country 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.