lOO A NATURALIST IN THE GREAT LAKES REGION 



The light relation determines not so much where the plant 

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

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

 They are found forming the ground stratum in the forest with 

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

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

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

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



lOO+X 



ABOIC rOttCST 



jMM55;^i7,:m''55V ' 



'■55-/.? 1 



SURFACE SURFACE 



SURFyiCF SURFJC€ 



DAMP FOREST qAK FOREST GLAOC 

 PRAIRIE MARGIN 



SuRFACe 



PUMAX FOREST 



RA VINE 



Fig. 62. — Diagram showing relative evaporating power of air as influenced 

 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 spleenwort,^5/?/gwzww angusti- 

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



