1 88 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



for the use of the tree than to the minimum Hght under which 

 it can still exist. In some species this optimum coincides with 

 the total daylight or with the conditions that surround a tree 

 grown in the open : in other species both the vegetative and 

 reproductive functions of the tree are most vigorous when the 

 amount of light is less than the total daylight ; that is, when 

 the trees are grown in a stand. 



ToleriDicc and Intolerance of Trees. — The ability of trees to 

 endure shade is called tolerance of shade, or often merely 

 tolerance. Trees that are capable of enduring shade are 

 tolerant; those requiring full light intolerant. Some species are 

 able to absorb enough light for assimilation even in the shade 

 of a forest canopy. Thus, hemlock and spruce spring up and 

 live for many years under other trees. Other species, such as 

 tamarack, aspen, grey birch, and most of the yellow pines, 

 require full light and cannot endure shade from above. All trees, 

 however, thrive in full light, especially if they have it from the 

 very start, and none requires shade except as a protection from 

 drying or from frost. This does not mean, however, that all 

 trees grown in the open absorb equally all the available light. 

 Even in full light they need and use differing amounts of light 

 for their best growth. This is accomplished by a definite 

 orientation of the foliage in relation to the source of light, by 

 the development of denser crowns which lessen the amount of 

 light that can penetrate into the interior, or by a change in the 

 structure of the leaves so as to decrease the assimilative energy. 

 In the open they are able to make these modifications for their 

 best growth to better advantage than in the shade of a dense 

 forest, and to adapt themselves to their needs; therefore, open- 

 grown trees usually appear the more thrifty. 



The primary cause of this difference in tolerance must be 

 sought in the anatomical structure and inherent qualities of the 

 leaves and the chlorophyll. Since, however, the anatomical 

 structure of the leaves and even the character of the chlorophyll 

 may be influenced by environment, the tolerance of trees is 

 not a fixed quality, but is subject to variation. Each species, 

 however, inherently requires a certain amount of light, which 

 cannot be changed by any environment. 



The latest experiments by Prof. Lubimenkoi established with 



' Carried on in 1904 al llie Laboraloirc dc Biologic Vcgelale, in Fontaine- 

 bieau, France. 



