so far as I am aware, the occurrence of both homogone and trimorphic species gives some reason for looking for still others which are dimorphic. In writing this I am perfectly aware that Hildebrand' has examined a few herbarium specimens of O. violacea, finding eight long-styled, three short-styled, and one midstyled plant in the twelve specimens examined. The constant lack of correspondence in our specimens between the sets of stamens which should correspond, however, leads one to wonder if a mistake may not have been made, especially since a slight discrepancy exists between the numbers cited and the summary, in the paragraph cited.

Both the long and short-styled flowers are visited by small bees in considerable numbers, the more common being Nomada bisignata, Ceratina dupla, Augochlora pura, an Osmia, and several species of Halictus. These insects are attracted by the nectar which is secreted, apparently, by the papillose bases of the petals, and which is protected from rain, &c., by pubescence on the styles in the long-styled flowers, and on the filaments in the other form. As a result of these visits, some flowers of both kinds produce capsules, which are by no means uncommon, although by far the greater number fall away without bearing any fruit.

:0:

FORESTS-THEIR INFLUENCE UPON CLIMATE AND RAINFALL.

THAT

BY J. M. ANDERS, M.D., PH.D.

THAT there exists some sort of relation betwixt forests and conditions of climate, perhaps most observers would be ready to concede. Many attempts have been made to explain how forests affect atmospheric states, but there is great diversity of opinion on the subject, and, indeed, the question to-day remains somewhat involved in obscurity. As every one knows, there was a time when forests were considered almost inexhaustible. It is also a well-known fact that the destructive hand of man began, centuries ago, to fell rapidly these abundant forests, and changes of climate and fertility of the soil have, in numerous regions, been attributed solely to this denudation of the land. On examining the literature of the subject, it is found that the balance of 'Monatsber. Berlin Akad., June 21, 1866, p. 357.

argument and opinion is decidedly on the side of the baneful effects of the destruction of forest growth, the testimony of some of the best scientific minds of different ages being very strong on this point.

It is but fair to say, however, that not a few observers of note deny any effects of woods on the moisture and other conditions of the atmosphere; and even stranger still, it has been declared that the climate of the Western States has, if anything, been improved by the denudation of forests; but this assertion rests, we think, on too slender evidence to be entitled to credence. It may be safely assumed that forests favorably affect the meteorological conditions. Our subject presents many difficulties owing chiefly to the fact that numerous causative elements enter into the question, some of which are of a conflicting tendency, and though a question so confessedly intricate may perhaps never be susceptible of solution, nevertheless any new light on the subject, however faint the ray, must be considered welcome.

One of the ways in which forests are usually considered to exert an influence over the climate, is by obstructing the free passage of wind currents. This is an element of the question which is, perhaps, better established than any other, but is of too great importance to be disposed of in a summary manner. It is evident that trees are well adapted to break the force of the wind; the branches, and particularly the leaves, on account of their immense numbers and close proximity, serve as efficient barriers, and the trunk holds up the bushy top and defies the tempest, while roots in turn are continually extending their grip on mother earth in order to support the trunk. And it can be readily understood that the particles not checked by the first row of trees to the windward, would have their force diminished and be promptly checked by the trees to the rear. In this wise belts or clumps of trees afford shelter to the leeward of them from the chilly, or even frigid blasts, which are known in many localities to be very unfavorable to the maturation of fruit-crops and harvests. Of little less importance, perhaps, is their effect in protecting from the drying winds of summer, which are frequently the cause of blighted crops and other mischief, due to their power to enhance evaporation from vegetation and from the soil during the dry seaFor this reason woods are also needed even on our coast. The sea breezes as they strike the land become warmed, their

son.

capacity for moisture is thereby increased, and naturally absorb with avidity the earth's moisture and produce a drying effect. It is plain to be seen then, that woods by intercepting cold currents. and drying winds, mitigate extremes-rendering summer less sultry and winter less severe, though they may not materially affect the mean temperature. In like manner they must tend to obviate the injurious consequences of cold spring and autumn winds, and thereby relatively lengthen the warm season or term of vegetable development. This is a highly important office, since some crops are slow in maturing.

The experiment has been tried extensively in France of planting trees in belts one hundred meters apart, and with marked benefit to the climate, and there are some good reasons for believing that a similar experiment in various places in our own country, would prove equally advantageous. It has been observed many times that fruit grown in the city surpasses in quality and size that grown in the country, and this is ascribable to the more effectual shelter in the former place.

The wind as it courses over an open country conveys with it a variable quantity of moisture, which, though usually invisible, is always present in the atmosphere, which is likewise arrested by the forest. Now what becomes of this moisture? The air is forced up by the side of the woods to the tops of the trees just as in the case of a low mountainous elevation, and owing to the attraction between its particles and the constant vis a tergo caused by fresh currents from behind, the volume does not stop here but rises higher. When the temperature of the air above is lower than that in the forest, as is sometimes the case when storms prevail, then there would also be an upward current from the tree tops. It is usually considered that in this manner forests increase the aggregate general rainfall, viz., by causing ascending currents to sufficiently high regions for the moisture to be condensed into clouds and rain, and this has been held by some to be the only way in which they influence precipitation. Meteorological science has, however, established the fact that rain is generally formed from one to two miles above the surface of the earth, and it would scarce be possible that an obstruction no higher than an ordinary forest could, per se, be capable of raising the vapor-laden air to this extent and could not actually increase the rainfall. On the other hand, when forests are situated on ele

vated ridges or mountains of moderate elevation, they may have the effect of extending the influence of the latter a step further in producing an upward current to the cooler regions, or condensing area, and in this manner greatly assist local precipitation. It is now a settled fact that high mountains augment the rainfall in themselves or even to some little distance from their bases. The Alps of Switzerland are known to modify and greatly influence the course of storms. We repeat it then, that forests resemble high altitudes as regards their mechanical action in affecting the rainfall, but owing to their meagre height, can scarce be said to have any influence (mechanically) over this phenomenon except they are situated upon the latter, in which case their action may tell considerably. Forests do, however, affect local precipitation through certain vital functions, as will be seen by and by.

No other influence which forests exert upon atmospheric conditions can claim so large a share of importance as that exercised on its humidity. The explanation of their effect on this meteorological element is to be found mainly in a study of some of the organic processes carried on by trees, but to a slight extent also to a mechanical action. The evaporation from the soil is interfered with by the vegetable canopy above, which prevents, in a great measure, the sun's rays from reaching the earth and heating it so as to facilitate evaporation. Again, by forming a more or less perfect screen interposed between sky and earth, forests in a measure intercept the dew and lighter rains, allowing but a portion of this moisture to reach the earth. It has been estimated that the evaporation from the soil of the forest is rather more than one-third as great as that from open soil, but this lessened surface evaporation is much more than compensated for by transpiration of the forest, as will be indicated by the results of our investigations.

The question of the influence of the organic functions of plants on the humidity of the air, is one of paramount importance and great philosophic interest. Whatever effect plants have through these processes must be due either to the exhalations of moisture from the leaves (transpiration) or to the absorption of moisture by the leaves. The latter idea, as formerly taught and until recently held by most authorities, is now most probably shown to be erroneous. According to the researches of Unger1 the theory of the absorption of the watery vapor by the leaves is untenable. My

1 Wilhelm der Baden und der wald, p. 19, quoted by Marsh.

2 The writer regrets that the details of these experiments are not accessible.

own observations tend to confirm the conclusions of Unger. A growing pot plant (geranium) in a thrifty condition was experinented with. The whole of the pot was covered with a double layer of oiled silk, and the free portion accurately adjusted around the base of the stem, on which it was tied with elastic cord. Thus prepared, no evaporation could take place from the soil in the pot, and what is of more importance still, no moisture could be thus supplied to the roots excepting that which was contained in the soil in the pot. The plant was now placed under a glass case which was situated over a shallow box in which there was about four inches of soil which was kept saturated so that the evaporation from it kept the air of the glass chamber quite moist. The whole arrangement was placed near a window with a southern exposure, the plant catching the rays of the sun for about five hours of the day in clear weather. In this situation. the plant remained quiescent or dormant so far as any visible growth or development was concerned, for about two weeks, when it began to look languid and the margins of the leaves began to change in color and to show slight signs of failing nutrition. The explanation of this apparently long state of hybernation in the plant is simple. The air in the case being too moist to allow of scarce any transpiration, the plant retained the moisture in the pot for purposes of nutrition only, and since the plant most probably grew but little during that period, there was quite sufficient water in the pot for its uses for so long a time. At the end of the two weeks the plant was taken out of the glass case and placed in a sick chamber with the same exposure, in which three dozen other thrifty plants were situated. The oil silk was allowed to remain on and no water was supplied to the roots of the plant. The atmosphere of the chamber was noticeably moist to the senses, though agreeable. Here the sun's rays had an opportunity of exciting the plant to transpire actively, and, as a consequence, in a few days nutritive change became very decided, leaf after leaf drying until at the end of another fortnight only a couple much withered leaves were left on the plant.

Now this experiment is not sufficiently conclusive to assure us that absorption of moisture by the leaves is impossible; but it certainly must show to the satisfaction of every one that not sufficient water can be taken in through them to carry on the normal functions of the plant, and renders it extremely probable the only source of moisture to the plants is through the roots.