388 
W. B. MCDOUGALL 
case the numeral is the number given to the individual tree, while A 
indicates a horizontal plate and B sl vertical plate. A + in the tables 
indicates that some elongation was observed in at least one root, 
while a — indicates that no growth was detected. 
Table I 
IQ14 Results 
3 A 3B 4A 4B 
sB 6A 
jA -jB 8/1 
Jan. 5 
Apr. 28 
May 5 
12 
19 
26 
June 4 
II 
20 
30 
July 7 
14 
21 
28 
Aug. 6 
13 
29 
Sept. 8 
16 
19 
^9 
Oct. 7 
15 
20 
31 
Nov. 7 
15 
24 
Dec. I 
7 
Discussion 
The cause of periodicity in plants has been the subject of much 
discussion. Schimper (7) concluded from his own observations, and 
the observations of other workers in tropical regions, that plants in 
general of necessity show periodicity, and that this is due to internal 
causes. Klebs (8), on the other hand, brought forward considerable 
evidence against Schimper's interpretation and denied the existence 
