LIGHT IN REI,ATION TO TREE GROWTH. l8l 



19. Light in Relation to Tree Growth. 



Bulletin No. 92 of the U.S. Forest Service, by Raphael Zon 

 and Henry Graves, entitled Light in Relation to Tree Growth, is 

 one of those interesting and valuable publications which show in 

 unmistakable fashion how properly organised investigations may 

 be of the highest importance in both pure and applied science. 



The Bulletin may be roughly divided into two parts. The 

 first part treats of the general phenomenon of light in relation 

 to plant growth. Special attention is drawn to the light 

 requirements of different trees, that is how the growth of 

 different kinds of trees is affected by light of different intensity. 

 The second part deals with the several methods, at present at 

 our disposal, of determining the light tolerance of various 

 species. The first part is of such importance and general 

 interest that it is here reproduced entire, since it would be 

 quite impossible to do it justice by means of extracts. The 

 author says : — 



Light is indispensable for the life and growth of trees. In 

 common with other green plants a tree, in order to live, must 

 produce organic substance for the building of new tissues. 

 Certain low forms of vegetable life, such as bacteria and fungi, 

 do not require light. They exist by absorbing organic substance 

 from other living bodies; but the higher forms of plants manu- 

 facture their own organic material by extracting carbon from 

 the air. The leaves, through the agency of their chlorophyll, 

 or green colouring matter, absorb from the air carbon dioxide, 

 and give off a nearly equal volume ot oxygen. The carbon 

 dioxide is then broken up into its elements and converted into 

 organic substances which are used in building up new tissues. 



Light is not only indispensable for photosynthesis, but it is 

 essential for the formation of chlorophyll itself. Only in excep- 

 tional cases, as in the embryo of fir, pine and cedar seeds, does 

 chlorophyll form in the dark, and, with the exception of some 

 microbes, the green cell is the only place where organic material 

 is built up from inorganic substances. 



Light also influences transpiration, and consequently the 

 metabolism of green plants. It influences largely the structure, 

 the form and the colour of the leaf, and the form of the stem 

 and the crown of the tree. In the forest it largely determines 



