1915] BROWN— PINUS STROBUS 



229 



to a demand for conducting tissue, while the late wood is formed to 

 increase the stability of the tree. The last factor may have been 

 in force from the beginning, but was at the start overshadowed by 

 the first. 



Mer's theory (23) rests firmly on the general assumption of 

 WiELER as given above. According to his idea, the early wood 

 results when the cambial activity is at a maximum, that is, in the 

 spring, while late wood formation occurs when growth is going 

 on very sluggishly, as in August and September in the white pine. 

 The last elements of the annual ring are flattened because with 

 the falling off of growth intensity the radial stretching of the 

 young elements subsides in the same proportion. 



Still another theory is of interest here because it departs 

 decidedly from all of those mentioned. Schwarz (31) assigns 

 the chief role in late wood formation to longitudinal pressure. 

 This is in force throughout the growing season, but its effects are 

 lost at first as the result of other factors, such as nourishment, 

 which are temporarily more potent at that time. With a decline 

 in the action of these, the effect of longitudinal pressure (gravity) 

 reasserts itself. 



No attempt has been made in the present work to refute or 

 substantiate any of the theories above mentioned, nor in fact to 

 bring forward a new hypothesis for annual ring formation. Other 

 workers of the last decade have given the matter serious thought, 

 but the problem still remains unsolved. It is the opinion of the 

 author that several factors are potential, but inasmuch as these 

 cannot be controlled by the investigator, the precise influence of 

 each on growth cannot be definitely determined. The results 

 obtained appear to substantiate Mer's theory to some extent, in 

 that growth in tree I was more rapid in the spring and early summer 

 than subsequently. But the assumption that the cambium was 

 better nourished at the beginning of growth, than later is not justi- 

 fied from the present inquiry. It can only be said, in conclusion, 

 that late wood formation occurs at a time when growth is proceed- 

 ing very slowly. 



No definite results were obtained concerning the time that late 

 wood formation begins. White pine does not lend itself to a study 



