FRAGMENTS OF SCIENCE. 



857 



the Bibliothdque, or Library of the Musee 

 Social. The Musee has been recognized by 

 the state as of public utility, and its mem- 

 bers and supporters are men of weight. 



Some Conditions of Plant Distribution. 



A paper published by Mr. Conway Mac- 

 Millan 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 environ- 

 ment, is very minute and exhaustive, and is 

 believed by the author to be the first of its 

 kind published in America. From the mul- 

 titude of illustrations it affords of the de- 

 pendence of vegetal and other character- 

 istics 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 excel- 

 lent example of the sensitiveness of plant 

 formations to varying environmental condi- 

 tions. The character and aspect, the abun- 

 dance or paucity of certain forms, the ar- 

 rangement 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 modi- 

 fications of these. So the constant variety 

 of the beach as one walks along it is con- 

 nected 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 af- 

 fected 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 de- 

 clivities or gullies in the back-strand, direct- 

 ing the moisture to some spots rather than 

 to others. Thus both the physical and bio- 

 logical conditions of the zone farther inland 

 affect the beach flora quite as distinctly as 

 do the conditions shoreward." The line be- 

 tween 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 devel- 

 oped ; 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 de- 

 serted it." 



The Bine Color of Lakes. It is general- 

 ly 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 sup- 

 posed 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 sup- 

 pose 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 experi- 

 ments 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 be- 

 tween the surface and the layers beneath are 

 greater. Thus the remains of the lake dwell- 

 ers 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 



