34 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
cells remained the same throughout the day.^ The greatest difference in 
osmotic concentration expressed in atmospheres was 5.97. It should be 
said in regard to the threshold concentration that it is difficult to determine 
exactly what concentration is the threshold, since all cells are not equally 
affected. The threshold concentrations in these tests were always con- 
sidered the points at which the slightest plasmolysis could be detected, 
For these reasons there is bound to be some error in the determinations, 
but when differences in the concentration are large there is little difficulty 
in making observations. It is worthy of notice that the difference between 
the guard cells and t he cells of the epidermis is very small as compared with 
the results secured by Iljin, although there is a rise with the opening of the 
stomata and a fall with their closing. The sun was visible 0.2 of the time 
from 8 to 9 a.m., and almost constantly from 9 a.m. to 12:30 p.m., after 
which there was no sunlight. 
Further observations were made on Zehrina pendula on November 29, 
December 5, and December 6. The results of these observations are also 
shown graphically in figure i, with the exception of those of December 5, 
when no differences were found. On November 29 the sun came out at 
9:30 a.m. and was fairly bright the rest of the day. On December 5 there 
was practically no sunshine throughout the day. On December 6 the sun 
did not shine until 9:30 a.m., after which there was almost constant sun- 
shine. On dark days there was very little if any opening of the stomata, 
although some stomata were always found open regardless of the time at 
which observations were made. On bright days there was a rather ready 
response of the stomata. They were usually open to their maximum by 
lo-ii a.m. 
No observations were made on this plant at Columbia, Missouri, because 
no vigorous plants were available. 
Cyclamen 
The first recorded tests on cyclamen were made on November 29, 
when the sun shone most of the time after 9 a.m. The results are given 
in figure 2. There was no great difference between the osmotic concentra- 
tion of the guard cells and that of the cells of the epidermis at any time. 
The greatest difference recorded was only a little over three atmospheres. 
The curve, however, indicates a rise in the osmotic concentration in the 
morning and a fall in the afternoon in the case of the guard cells (observa- 
tions made at Ithaca). 
The second series of observations was made on December 6, when the 
sunshine was rather constant after 9:30 a.m. These results are in almost 
perfect accord with those given for November 29 (observations made at 
Ithaca). 
2 The threshold concentration of the epidermal cells was found to be constant in all the 
experiments. Only slight variations in the readings occurred in every case. For this 
reason the threshold concentration of the epidermal cells is given as a straight line in the 
graphs. 
