Books and Current Literature. 



211 



silvical conditions observed in our American forests, and comes 

 at once to the important question : To what extent have the 

 effects of a deficient supply of soil moisture been ascribed to 

 a deficient supply of light.-' 



Discussing the well-known habits of growth and the struc- 

 tural peculiarities of trees grown in the open, the author holds 

 that these are plainly referable to light conditions, but says, 

 " We shall come nearer the heart of the matter if, instead of try- 

 ing to draw a final conclusion from the presence or absence of 

 this or that structural feature of the leaf, we think rather of the 

 physiological activity which is going on within it.'* 



Narrowing the question to the supply of light and the supply 

 of nutrient soil-water, it is evident that there must be for both 

 of these requirements of the plant a certain optimum, any falling 

 off from which must be accompanied by a proportionate abate- 

 ment of activity With a falling off in the supply of either water 

 or light, the work begins to grow languid. Under these circum- 

 stances it may well be that either an access of water or a mod- 

 erate access of li<ht would stimulate leaf activity and. in con- 

 sequence, growth of the tree; for either would stimulate 

 transpiration, while more light, would in addition, furnish more 

 utilizable power. 



Such an assumption appears most consistent with the facts 

 as we know them. A moderate thinning benefits a crowded 

 stand, but too heavy a thinning endangers it. The thinning 

 relieves the competition of the roots for soil-moisture, and at 

 the same time brings the stimulus of more light and results in 

 conditions which make evaporation more easy. A half-starved, 

 feeblv active organism, which has let down its guards against 

 giving off water too fast — has let its safety-valves, so to speak, 

 get out of order — all at once receives the call to leap into full 

 functioning. In spite of the fact that there is probably more 

 water available for it than before, the chances that it will die 

 for lack of water are decidedly increased. And it is worth ask- 

 ing, too, whether the roots have not shared in the general let- 

 down of activity during partial suppression so that they can not 

 at once get to work at full speed. 



Admitting that this and other considerations advanced by 

 the writer are, as yet, largely theoretical, they are, nevertheless, 



