THE GROWTH OF FOREST TREE ROOTS 
Table II 
IQ15 Results 
Date 
Acer sac- 
charinum Plates 
Carya laci 
niosa Plates 
Qnerctis a 
lia Plates 
■zA 
2B 
5^ 
6A 
7A 
7B 
8^ 
Mar. 23 
Apr. 5 
+ 
+ 
+ 
— 
— 
+ 
— 
— 
— 
— 
13 
+ 
+ 
+ 
+ 
— 
+ 
+ 
+ 
— 
+ 
20 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
26 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
May 8 
+ 
+ 
+ 
+ 
+ 
+ 
17 
+ 
+ 
+ 
+ 
+ 
+ 
-i- 
+ 
+ 
25 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
June 3 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
12 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
12 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
26 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
July 5 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
20 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
27 
+ 
+ 
+ 
+ 
+ 
+ 
Aug. 3 
+ 
+ 
+ 
+ 
+ 
+ 
,10 
+ 
+ 
+ 
+ 
? 
+ 
+ 
+ 
+ 
+ 
18 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
25 
+ 
+ 
+ 
+ 
+ ? 
+ 
+ 
+ 
+ 
+ 
Sept. 2 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
of periodicity independent of external factors. If the cause of a 
periodic phenomenon is internal, then, of course, there can be nothing 
gained by searching for a cause in the external environment. On the 
other hand, if the cause is external, then it is surely capable of being 
discovered, and, if an adequate cause is found in the external environ- 
ment, then there is certainly no need of presupposing any mysterious 
internal factor. 
Much evidence might be cited in support of the view that per'o- 
dicity is due to external factors. It 's well known that the resting 
period n various plants can be shortened by etherization or other 
means, and that many plants which, under natura conditions, have 
well-marked resting periods, may by artificial means be made to grow 
continuously. Appleman (9) has shown that the rest period of 
potato tubers is not of internal origin, but is dependent on external 
factors, the most important of which is the oxygen supply. Ac- 
cording to Brown (10) the termination of the latent period in Pinus 
strohus is dependent upon three external factors; moisture, temperature 
and available reserve foods. 
The results recorded in the present paper show conclusively that 
