THE PLANT WORLD. 
184 
examined at the same hour. The similarity is much closer than 
was to be expected on account of the numerous factors liable to 
cause individual variations. At 9 and 11 a. m. and 1 p. m. the 
numbers of dividing cells are almost identical in two separate 
tips. Some differences occur at 5 and 9 p. m. and 1 a. m. The 
curves deduced from the tables are remarkably regular, there 
being two maxima and two minima. Cell-division is most rapid 
(primary maximum) at 11 p. m. The secondary maximum 
occurs at 1 p. m. The primary minimum in cell-division occurs 
at 7 a. m. following the primary maximum. The secondary mini¬ 
mum occurs at 3 p. m. following the secondary maximum. 
Thus there are two regular waves of cell-division during the 
24-hour period, a more extensive wave extending from late in 
the afternoon until early in the morning and occupying therefore 
practically all of the dark period; its crest appearing shortly after 
midnight. A less extensive wave occupies the light period of the 
day, extending from early morning until late in the afternoon. 
The crest of the wave appears about noon. Both of these waves 
are remarkably regular. It was found that when the usual rhythm 
of cell-division is interfered with, the normal rhythm tends to 
recur when the disturbing factor is removed or overcome. 
Roots of Podophyllum, studied in precisely the same way, gave 
similar results. In general the character of the curves for this 
plant is much like that of the curves for Allium, except that there 
were more extensive minor variations. The maximal and min¬ 
imal points occurred at approximately similar hours with Allium . 
The general result of placing roots in solutions, as found by the 
use of water and glucose and magnesiutn chloride solutions, is to 
decrease the rate of cell-division and to delay the normal rhythm. 
It is noteworthy that in none of the series could there be seen 
any direct effect of change of temperature. Nevertheless there 
can be no doubt that wide variations in temperature would have 
a marked effect upon the rate of mitosis. 
The daily periodicity of elongation was studied by direct obser¬ 
vation. Onions were grown in a dark room in pots with a nar¬ 
row glass slip set in one side. Roots frequently grew almost 
vertically down along this glass and were observed by candle 
