ADAPTATIONS FOR SECURING LIGHT 125 



Not all trees can be classified as very tolerant or intolerant since 

 some species, such as the white oak {Quercus alba) and the white pine 

 (Pinus stroh7(s), are intermediate in this respect, that is, they can 

 indure some shade but not so much as such trees as sugar maple and 

 hemlock. The relative tolerance of species can often be determined 

 fairly accurately merely by observing their manner of growth and 

 degree of success under varying conditions. More accurate results 

 are obtained by measuring the light in various forests where the 

 seedlings are growing or fail to grow. The minimum amount of 

 light necessary for continued growth varies from about 2 per cent of 

 full sunlight for very tolerant species to about 25 per cent for in- 

 tolerant species. A still more accurate method is to actually grow 

 the species under varying amounts of controlled shade but this 

 method involves a long period. of time. In using any of these 

 methods, however, it must be remembered that light is not the only 

 factor involved in tolerance. Such factors as soil fertility and tem- 

 perature are often very important and soil moisture is often more 

 important even than light. 



76. Adaptations for Securing Light.— Plants are adapted in vari- 

 ous ways for procuring the necessary amount of light. Our early 

 spring flowers which live in forests, for example, carry on those 

 activities that require an abundance of light before the trees have 

 leaved. INIany of them are active only about three months and, be- 

 coming dormant soon after the leaves of the trees are out, remain so 

 until the following spring. The habits of twining plants and of other 

 climbers are significant primarily because they serve to bring the 

 foliage into such positions that it may be displayed to the light. 



The foliage of many plants has a mosaic arrangement, often 

 called a leaf mosaic, in which the leaves are arranged in such a way 

 that there is a minimum of overlapping, consecutive leaves never 

 being directly above one another. In many trees the branches are 

 progressively longer from the top toward the base so that the leaves, 

 which occur only near the outer extremities, are not shaded by the 

 leaves of higher branches. This is especially true of coniferous trees 

 which are often nearly perfectly cone-shaped but it is more or less 

 true of many other trees and also of many herbaceous plants. When 

 trees grow close together as in a forest the lower branches which 

 receive insufficient light soon die and drop oft' (Fig. 60). This is 

 especially true of intolerant trees. Such trees are no longer cone- 

 shaped, but their leaves are produced only near the top and so are 

 all well exposed. 



