LIGHT IN RELATION TO TREE GROWTH. 



189 



sufficient accuracy the difference in the sensitiveness of the 

 chloroplasts of different tree species. The species with which 

 he experimented were Scots pine {Pi/ii/s sy/vesfris), noble fir 

 {Abies //obi/is), wliite birch {Betula a//>a), and linden {Tilia 

 cordata). Of these, pine and birch are light-needing species 

 and stand close to each other in this respect. The fir and 

 basswood (or linden) are classed as tolerant, and are also fairly 

 similar in their demands upon light. Moreover, the anatomical 

 structures of the leaves of pine and fir are very similar, and of 

 birch and basswood practically identical. 



These experiments brought out clearly the following points : 



1. The initial light intensity at which assimilation begins 

 varies with the species, since the fir and basswood began 

 assimilation at r/5 of the light intensity at which pine and birch 

 began to assimilate. 



2. If it be accepted that the light intensity at which assimila- 

 tion equals respiration is the minimum for the existence of the 

 leaf, then birch appears to be the most light-demanding species, 

 followed by pine, basswood, and finally fir. 



3. With increase in light, the assimilation at first increases in 

 all four species, but in direct sunlight, when the rays strike the 

 surface of the leaves perpendicularly, the pine and birch still 

 continue to show an increase in assimilation, while the fir and 

 basswood show a decrease. This may be seen in Table II. 



Table II. — Relative amoinits of carbon dioxide absorbed by i gram 

 of leaf during one hour of 7Uork. 



Since the anatomical structures of the leaves of pine and fir 

 are very similar, and of birch and basswood almost identical, 

 this difference in assimilation must be explained exclusively by 

 the difference in the sensitiveness of their chloroplasts. These 

 figures show also, in a general way, that a unit (by weight) of 

 leaf substance of broad-leaf species assimilates considerably 

 more than the same unit of leaf substance of coniferous species. 



