Page:Popular Science Monthly Volume 51.djvu/875

Rh the Bibliothèque, or Library of the Musée Social. The Musée has been recognized by the state as of public utility, and its members and supporters are men of weight.

Some Conditions of Plant Distribution.—A paper published by Mr. Conway MacMillan in Minnesota Botanical Studies on the Distribution of Plants along Shore at Lake of the Woods is an admirable demonstration of the dependence over such an area as the shores of the lake of plant formations upon conditions of topography and the environment, is very minute and exhaustive, and is believed by the author to be the first of its kind published in America. From the multitude of illustrations it affords of the dependence of vegetal and other characteristics on small differences of conditions, we can take, almost at random, only a few. The shores of the lake are classified into front, mid, and back strands, etc. "The mid-strand area appears to afford an excellent example of the sensitiveness of plant formations to varying environmental conditions. The character and aspect, the abundance or paucity of certain forms, the arrangement of the different forms with reference to one another, all seem definitely related to the variations in exposure, slope, temperature, moisture, wind currents and surf impact, or upon combinations or modifications of these. So the constant variety of the beach as one walks along it is connected with the multitude of variations in the soil below, the air above, and the water off shore. The mid-strand, too, is modified by the back-strand which abuts upon it. . . . And by the physical texture and contour of the back-strand the mid-strand may be affected very sharply—as when the rain is carried through the gullies in the back-strand down upon or across the area nearer the water's edge. Curious interrupted patches of Carices and Epilobiums that occur in the mid-strand are often to, be referred to declivities or gullies in the back-strand, directing the moisture to some spots rather than to others. Thus both the physical and biological conditions of the zone farther inland affect the beach flora quite as distinctly as do the conditions shoreward." The line between mid-strand and back-strand may be called a tension line, as between two general groups of plants striving to move in opposite directions, where a reciprocal stress is developed; and the plants of the mid-strand strive to enter the back strand, while the others try in turn to work out upon the mid-strand. Thus an irregular boundary line is developed, and the exact line of demarcation is nowhere altogether clear and distinct. A peculiar biological influence modifying back-strand at certain isolated points is the nesting of gulls and terns. "By their deposition of guano, and probably by their carrying in of seeds, these birds have at various points on island back-strand established conditions favorable to the development of vegetation islands that may mark the approximate spot of the rookery long after the birds have deserted it."

The Blue Color of Lakes.—It is generally agreed, as Carl Vogt demonstrated in an article published in the Monthly a few years ago, that pure water, as in many of the deepest lakes, is blue; and it is usually supposed that the greenish tint common to other waters is given to them by yellowish matter held in suspension, while an excess of such matter turns them yellow. The explanation, while he regards it as correct as to the color of water, is not accepted by M. W. Spring as sufficient to account for lakes looking blue; for, if their water is wholly pure and quiet, it will absorb the mass of the light, reflecting little or none, and look black. What gives this water its reflecting power? Some suppose the existence of colorless solid matter in the water like the dust that makes visible the diffused light of the atmosphere. That cause is admitted to be a possible one; but M. Spring has satisfied himself by experiments that water absolutely pure will also reflect the light if the mass is composed of layers of different temperatures that give rise to convection currents. This conclusion is supported by observation. Prof. F. A. Forel has found that fresh-water lakes are more transparent in winter than in summer, as they should be by M. Spring's theory; because in summer the differences in temperature between the surface and the layers beneath are greater. Thus the remains of the lake dwellers can be seen on the bottoms of the Swiss lakes in winter at places where they are not at all visible in summer. Prof. Forel thinks