RESEARCH METHODS IN STUDY OE FOREST ENVIRONMENT. 39 
SOLAR RADIATION— LIGHT. 
The importance of light, at all stages in the development of trees, 
has never been underestimated by foresters. On the contrary, re- 
viewing the literature of forestry at the present time, it would seem 
that this element of the environment has been emphasized, by some 
almost to the exclusion of all the other conditions. It was. perhaps, 
only natural that casual observers of the forest should mention 
this factor more frequently than all others, because the presence 
or absence of light is so easily detected. It must now be admitted, 
however, that visible light does not tell the whole story : and. fur- 
thermore, that phenomena, commonly called by foresters the " sup- 
pression " of trees, which have often been credited to insufficient, 
light, may be and probably are in many instances caused by lack 
of moisture. 
In this country Zon (77) citing experiments of Fricke. was one 
of the first to call attention to the relatively great importance of 
factors other than light. It may be that his suggesion created too 
strong a reaction, that there has been too much of a tendency on the 
part of American investigators to ignore light or to be satisfied 
with an incomplete study of its ecological relations. It is believed. 
however, that this is only apparently the case, the situation being 
explained by the large amount of ecological study that has been per- 
formed in the western mountain forests, where sunlight is not defi- 
cient and precipitation or soil moisture appears usually to be the 
more vitally controlling factor. 
On the other hand, the work of Burns (56-59) shows substantial 
progress in the study of light. Its effects on growth have been 
directly observed, and its bearing on the transpiration rate and 
water requirements of young trees is. at least, strongly suggested. 
Pearson (12). Clements (60). and many others have made observa- 
tions on light under less controlled conditions. The principal lesson 
to be learned from the progress to date, however, is that light can 
not be taken independently without regard for other conditions 
that it modifies and all of the plant functions which it stimulates. 
Xowhere. perhaps, is a better illustration found of the danger of 
one-sided ecological investigations than the common error of forest- 
ers in ascribing all bad effects of crowding in the forest to lack of 
light. 
Concept of the Functions of Radiant Energy. 
The solar radiation available to the plant not only supplements 
the heat available by conduction from the air but is vitally necessary 
to the chemical activities of the plant, of which photosynthesis is 
foremost and of most direct interest to the ecologist. It is fairly 
evident that sunlight has an influence on the temperature of leaves 
