G12 JOURNAL OF FORESTRY 



Physiologists agree that the specific assimilatory energy varies with 

 different chloroplasts. Zon transfers this thought to forestry when 

 he says that "each species requires a certain amount of light which 

 cannot be changed by environment." Lubimenko attempted to show 

 that this variation on the part of the chloroplasts was sufficient to 

 account for the succession of trees. He found that the initial light 

 intensity at which photosynthesis begins in the case of two tolerant 

 trees — fir and basswood — was only one-fifth the intensity required by 

 intolerant trees — birch and pine. To him tolerance means a definite 

 thing, namely, the difference in sensitivity of the chlorophyll bodies to 

 white light. It was a splendid piece of work attempting to give his 

 theory of tolerance a scientific basis. However, his source of light 

 was a gas flame, and too great weight cannot be placed on his results. 



I know of no work of this kind dealing with the maximum light 

 which trees can endure, but the literature of plant physiologists and 

 foresters states that the diffuse and not the direct light is the most 

 important for photosynthesis. Ewart has shown that too strong a 

 light may be an inhibiting factor in food manufacture. 



One of the standards used in judging trees as to tolerance was 

 stated as the structure of the leaf — its position, size, and compactness. 

 The work of Stahl has shown that these leaf structures may be influ- 

 enced by light, and he speaks of sun and shade leaves. These terms 

 are familiar to us all. However, Transeau, working with some 

 shrubby and herbaceous forms, was able to develop shade leaves and 

 sun leaves by controlling the temperature of the soil by means of coils 

 of glass tubing through which ran iced water. He says, "All of these 

 xeophilous characters may be produced by growing the plant in an 

 undrained wet sphagnum substratum whose temperature is maintained 

 a few degrees below that of the air. This effect is obtained even in 

 subdued light." In this work palisade is to be associated with drought 

 rather than light. If this work had been done first we might have been 

 speaking of warm soil leaves and cold soil leaves instead of sun and 

 shade leaves. In fact we find in the literature the suggestion that our 

 tolerant trees are hydrophytic and our intolerant trees are xerophytic, 

 and it has been suggested that these two words supplant tolerant and 

 intolerant. 



This would make light, if we accept Stahl's work, only an indirect 

 cause of tolerance. However, this work of Transeau's cannot be ig- 

 nored, and it suggests a field which foresters must work out in refer- 



