DAILY RHYTHMS IN CERTAIN ROOTS 
during the winter months from material collected outside. Kurssanow 
(17) reports Zygnema as dividing most frequently between 9 P.M. and mid- 
night. Numerous other examples from the older literature are cited by 
Karsten (12) which will not be reproduced here. Karsten (13) in the most 
recent paper has shown that the desmids: Cosmarium Botrytis, Closterium 
moniliferum, and Mesotaenium Endlicherianum, when grown under normal 
conditions of illumination, exhibit a regular daily periodicity in the rate of 
nuclear and cell division. Cosmarium exhibits three waves. The primary 
maximum (about 50 percent of all cells) occurs at i A.M., with secondary 
maxima at 5 and 11 A.M. The primary minimum (about 5 percent of all 
cells) occurs at 1-3 P.M., with secondary minima at 3 and 7 A.M. Similarly, 
Closterium and Mesotaenium exhibit regular waves in the percentage of 
cells undergoing division, differing only in detail from the condition above 
noted for Cosmarium. It should be borne in mind that all of the above 
cited cases are reported from experiments carried on under normal conditions 
of light and darkness. 
Aerial parts of higher plants. The only published reports on periodicity 
of cell division in aerial parts known to the writer are those of Karsten 
(12 and 13). He used as material the apical meristem of seedlings of Pisum 
sativum, Zea Mays, and Pinus austriaca. Seedlings of Pisum grown in 
continuous darkness showed a very marked increase in the number of cells 
undergoing division between 9:30 P.M. and 2 A.M., with a minimum falling 
at 6 A.M., while the remainder of the day was occupied with smaller fluc- 
tuations. Similarly, the curve for Zea Mays grown in continuous darkness 
shows numerous minor oscillations during the day, with a very marked 
rising during the night until the crest is reached at about 4 A.M., from which 
time it falls back again to the day position. This rhythm is ifidepejident of 
changes in illumination and temperature. The effect of alternation of dark- 
ness and light was then studied. When plants were lighted during the 
day and darkened during the night, much the same sort of curve was 
obtained as when in continuous darkness. When the times of illumination 
and darkness were reversed, two waves appeared with maxima at 6 A.M. 
and 6 P.M. and minima at 4 P.M. and 10 P.M. When the plants were 
continually lighted the waves were much shorter and more numerous. 
Seedlings of Pinus austriaca, when grown under normal conditions, showed 
maxima at 4 A.M. and 4 P.M. and minima at 12 M. and 6 P.M. 
Underground parts of higher plants. The earliest work of this sort done 
on roots is that of Lewis (21). In the preliminary notice of this work it is 
shown that roots from bulbs of Allium Cepa, when grown in water and 
under normal conditions of illumination, i.e., regularly alternating day and 
night, show two waves in their rate of cell division. The maxima come at 
midnight and noon, and the minima at 4 A.M. and 4 P.M. When yellow 
light was used the maxima appeared as before, but with the minima at 
8 A.M. and 8 P.M. In blue light the maxima occurred at 4 A.M. and 
