THE GROWTH OF FOREST TREE ROOTS 
W. B. MCDOUGALL 
It has become a commonly accepted view that the growth of tree 
roots takes place in spring and in autumn, and that there is a period 
of rest in mid-summer as well as in winter. The workers who have 
published on this subject, however, do not all agree as to the time of 
year of the growth and rest periods, and no attempt seems to have 
been made by any of them to determine to what factor of the environ- 
ment, if any, a summer rest period might be due. 
Resa (i), who was the first to work intensively on the subject, 
came to the conclusion that deciduous-leaved trees produce their new 
roots in the autumn, while coniferous trees produce them in both 
autumn and spring, the exact time depending upon the weather. In 
the deciduous-leaved trees he found some root growth in the spring 
but not the formation of new roots. He also stated that the autumn 
growth period persisted, to a certain extent, throughout the winter, 
the period of root growth not corresponding at all with that of aerial 
growth. Wieler (2), on the other hand, found root growth only in the 
spring; but Petersen (3), Hammerle (4), and Biisgen (5), all agreed 
with Resa that growth occurs in autumn as well. 
Methods 
The work on which the present paper is based was carried on in 
the ''Forestry," a small artificial wood-lot at the University of Illinois. 
Two methods, or rather two modifications of one method, were used 
for making observations on the same roots at intervals throughout the 
growing season: (I) The horizontal glass-plate method. This meth- 
od is similar to that used previously by the author (6) in making 
direct observations on developing mycorrhizas. The leaf mold and 
humus were scraped away to a depth of about two inches or until 
some healthy roots were exposed. These roots were then covered 
with a square of window glass, one foot square. Over this was 
placed a square of felt roofing and the whole was then covered with 
soil. The roofing kept the glass fairly clean but, nevertheless, it was 
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