100 A NATURALIST IN THE GREAT LAKES REGION 



The light relation determines not so much where the plam 

 grows as its habits of growth in a locality fixed chiefly by th( 

 water supply. Thus, some plants are said to be shade loving 

 They are found forming the ground stratum in the forest witt 

 other plants, a shrub stratum, over them, plants less able tc 

 endure the shade, though they in turn are overgrown by a tree 

 stratum whose members insist on getting up into the full glare 01 

 the sunlight (see Fig. 297). The entire association is due to the 



IQO+X 



FIG. 62. Diagram showing relative evaporating power of air as influencec 

 by prairie and forest vegetation. After Adams. 



mesophytic conditions that all need, and probably the stratifica- 

 tion is due quite as much to the relative amounts of evaporation 

 (see Fig. 62) as to the varying light intensity. How dim the 

 light is in the ground layers of the forest becomes apparent in try- 

 ing to take pictures in the beech-maple woods; the exposure meter 

 indicates an intensity one-twentieth that of the surrounding 

 open pastures; so undoubtedly the light factor is to be regarded 

 as. of great importance. This is made more evident when the 

 fact is recognized that the same shade-loving plants found on 

 the forest floor are also often found in the rock ravine. Thus, the 

 clearweed, the touch-me-not, and such characteristic ferns as the 

 beech fern, the fragile fern, and the spleen wort, A splenium angusti- 

 folium, are common in both locations (Figs. 270, 273, 274, 395). 



