74 DEVELOPMENT AND HEREDITY. 



object under experiment be placed in darkness 

 with unchanging conditions of temperature and 

 humidity. In this way, as is known, it is possible 

 to prove that the flow of sap, which can now be 

 observed in the glass tube, shows a periodical 

 alternation. The root-pressure does not always 

 drive out, from the cut end of the stump, the same 

 amount of fluid in a given unit of time ; but a 

 variation of the sap-flow makes itself apparent in 

 such a way that in general the maximum flow 

 occurs in the afternoon hours, and the minimum 

 flow occurs in the early morning hours. The sap- 

 flow may continue for days ; and Baranetsky and 

 I each concluded from our investigations of the 

 root-pressure, that the cause of the daily periodicity 

 of root-pressure lies in the daily and nightly alter- 

 nation of light and darkness to which the plants 

 were exposed before the experiment. In support 

 of this view, for example, is the fact that when the 

 tops are cut off of etiolated plants which have been 

 raised in complete darkness, the sap under the 

 pressure of the roots continues to flow from 

 the stump, but no periodicity can be detected in 

 the flow. 



" It is a well-known fact that darkness has a 

 quickening effect upon the growth of plants, while 

 illumination makes the growth slower. When, 



