268 



ESMOND R. LONG 



the next morning. Similar calculation for all the plants exam- 

 ined gives the results shown in table 5 for the varying water 

 content. 



If we assume that at a given time of day two halves of the plant 

 are similar in water distribution, and this is probably fair in view 

 of the symmetry of these plants, then the above figures simply 

 show that generally the difference between the water content 



TABLE 4 



Showing differences in the acidity of sap in samples taken in afternoon and morn- 

 ing at different depths from plants of Echinocactus collected June 30, July 8, and 

 July 18 



of the external and internal simply diminishes during the night. 

 During the day water evaporates from the surface more rapidly 

 than it can be supplied by conduction and diffusion from the in- 

 ternal parts. The lost water of the surface layer is replaced 

 again at the expense of the interior (which by the excision of the 

 plant was cut off from its water supply) during the night. Thus 

 we may conclude from the general observation that the water 



