1891.] The Growth-Periodicity of the Potato-Tuber. 467 
ticular, and those of the fifth day likewise, except that the latter 
showed a less maximum growth. The sixth day was péculiar, 
During this day no appreciable drop in the tracings was detected. 
The explanation of this cessation is not offered. It may be said, 
however, that the death of the top was now about complete, so 
far as the leaves and the secondary branches were concerned. 
Only in the lower part of the main stem was living green tissue 
to be found. During the whole twenty-four hours of the sixth 
day, then, no divergence of the tracings from the horizontal was 
observed ; but during the succeeding twelve hours a slight drop 
began. At 7 o'clock a.m. of the seventh day a decided 
drop began, continuing until 11 a.m. There then succeeded a 
period of gradual dropping, which disappeared about 3 P. M. 
Another drop took place in the evening from 6 tog p.m. The 
eighth day began with a drop at 7 A. M., continuing until 11 A. M., 
when three hours of horizontal marks followed. At 2 P.M.a 
five-hour drop began, and continued as a gradual depression until 
IO P.M. At 7 A.M. again another abrupt drop was registered, 
terminating at 11.30 A.M. At 3 P.M. a gradual, slight drop, last- 
ing until 8 P. m., ensued. During four succeeding days the same 
thythm continued, only the drops became slighter and slighter, 
Finally, on the fifteenth day the needle ceased to trace. 
The explanation of these very curious maxima and minima in 
the growth of the tuber isa complicated matter. -It can be given 
as yet only conjecturally. Before passing to any such conjectures 
it may be well to give in their order the conclusions arrived at 
from the line of research described above : 
- 1. The increase in diameter of the potato-tuber is not regu- 
lar, but is rhythmic. 
2. Maxima of growth may occur either once or twice, and 
perhaps oftener, during twenty-four hours. 
3- Maxima of growth are not of long duration, and are fol- 
lowed by periods of slower growth, or of entire cessation of 
4. The maxima of some days are greater absolute maxima 
than those of other days. This indicates a grand-period for the 
tuber. 7 : 
