260 
LOCK : THE GROWTH OF 
5 .—The Daily Periodicity of Plant Functions . 
The daily periodicity exhibited by many of the functions 
of plants has long excited considerable interest. Pfeffer* 
definitely associates such a periodicity in the case of growth 
with the daily alternations of light and darkness. This 
association rests principally upon the observations of Sachs 
and of Baranetzky. And for the relatively small number of 
plants with which they worked such a conclusion seems to 
have been well established, although it does not appear quite 
certain in either case that periodic changes in the psychro- 
metric condition of the atmosphere were entirely excluded. 
Baranetzkyt records no observations upon the psychro- 
metric conditions of the air. And although the atmosphere 
of a closed room does not exhibit any such marked changes 
in this respect as does the free air outside, yet the sugges¬ 
tion arises that changes of moisture may have had some 
influence upon the marked irregularities in periodicity which 
appeared in Baranetzky’s plants. On the other hand this 
author states that sprinkling the floor with water had no 
effect upon the growth of the plants. 
Baranetzky ascribed every case of periodicity observed by 
him to the action of alternating light and darkness. Those 
which appeared in continued darkness were explained as 
being due to an after effect ; and in some cases such an 
effect seems to have been clearly demonstrated. 
The case of Brassica Rapa is more difficult. Etiolated shoots 
of this plant which had developed in the dark from tuberous 
roots, and were never exposed to light, showed never¬ 
theless a clear daily periodicity. The plants exhibited—as 
clearly appears from Tables XXIX. and XXX.J—a marked 
tendency to an increase of growth by night and a falling off 
during the day. The temperature during these experiments 
* Pflanzenphysiologie, Bd. II., p. 252 
f Baranetzky—Die Tägliche Periodicität im Langenwachsthum der 
Stengel. Mem. Acad. Imp. St. Petersburg. 1879. 
% P. 79, 80, loc. at. 
