THE GROWTH OF FOREST TREE ROOTS 
usually necessary to remove the glass when an observation was to 
be made. (II) The vertical glass-plate method. In order to 
make observations on roots located somewhat deeper than was 
possible by using horizontal plates, holes were dug in the earth two 
and one-half feet wide by five feet long and two feet deep. A 
glass plate, one foot square, was then placed against the soil and roots 
at one end of the hole in such a way as to exclude any extensive air 
spaces between the glass and the soil, and held in place by means of 
long pins made of number nine wire. A piece of felt roofing was 
placed against the glass to exclude light and was held in place by 
board props. The hole was then covered with a board cover hinged to 
two stakes at one side and locked with a padlock to a third stake at the 
opposite side. 
Observations were made by both of the above methods on two in- 
dividuals of each of the following four species of trees: Acer sacchar- 
inum L., Tilia americana L., Carya laciniosa (Michx f.) Loud., and 
Quercus alba L. The holes were dug and the glass put in place during 
December, 1913. They were first visited for observation on January 
5, 1914, but regular observations did not begin until the twenty- 
eighth of April of that year. The work was continued until September 
2, 1915. During the warmer parts of the year observations were 
made weekly except on two occasions, one in August, 1914, and one in 
July, 1915, when I was out of town for periods of two weeks. During 
the winter, observations were made usually only when the soil was not 
frozen. During the warmer part of the summer it was found that 
observations could not be made during the middle of the day without 
the roots being injured or killed by exposure to the warm dry air, 
especially on clear days. For that reason observations were regularly 
made just at or just after sunrise. In spite of this precaution many 
rootlets had their growth checked or even stopped permanently by 
exposure while observations were being made. 
At each observation a chart of the observation field was made and 
the position of each fresh-looking rootlet was indicated thereon. At 
the same time the length of each rootlet was measured and recorded. 
Growth was recorded only when an increase in length could be de- 
tected in a week's time by measuring with an ordinary millimeter rule. 
