1915] BROW N—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. Scuwarz (31) assigns 
the chief réle 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 Mrr’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 
