^Q THE HABITAT 



sun, and the intensity constantly swings from one extreme to the other. 

 Moreover, the extremes i.oo and 0.17, even if constant, are hardly sufficient 

 to produce a measurable result. When the angle of the leaf approaches 90, 

 there is the well-known differentiation of leaf surfaces and of chlorenchym, 

 but this has no relation to the angle of incidence. 



Table of Intensity at Various Angles 



In the sunlight, it makes no difference at what height a light reading is 

 taken. In forest and thicket as well as in some herbaceous formations, the 

 intensity of the light, if there is any difference, is greatest just beneath the 

 foliage of the facies. In forests especially, the light is increasingly diffuse 

 toward the ground, particularly where layers intervene. In woodland for- 

 mations, moreover, the exact spot in which a reading is made must be care- 

 fully chosen, unless the foliage is so dense that the shade is uniform. A 

 very satisfactory plan is to take readings in two or more spots where the 

 shade appears to be typical, and to make a check reading in a "sunfleck," 

 a spot where sunlight shows through. In forests and thickets, the sunflecks 

 are fleeting, and the light value is practically that of the shade. In pass- 

 ing into open woodland and thicket, the sunflecks increase in size and per- 

 manence, until finally they exceed the shade areas in amount and become 

 typical of the formation. 



Reflected and Absorbed Light 



88. The fate of incident light. The light present in a habitat and incident 

 upon a leaf is not all available for photosynthesis. Part is reflected or 

 screened out by the epidermis, and a certain amount passes through the 

 chlorenchym, except in very thick leaves. The light absorbed is by far the 

 greatest in the majority of species. Many plants with dense coatings of 

 hairs reflect or withhold more light than they absorb, and the amount of 

 light reflected by a thick cuticule is likewise great. As light is imponder- 

 able, the actual amount absorbed or reflected by the leaf can not be deter- 

 mined. It is possible, however, to express this in terms of the total amount 



