a 
24 
below, the hinge responds before the cells of the green mesophyll begin 
to suffer. The explanation is as follows: All the cells hold their water 
through the osmotic activity of their contents; the turgor of the 
chlorophyll-bearing cells is such that plasmolysis begins in about 0.5 nor- 
mal potassium nitrate, while the cells of the hinge begin to plasmolyse in 
less than 0.3 normal, as a consequence the latter will lose the greater 
amount of water in the shorter time, thus causing the hinge to close. 
The actual behavior of the hinge is sufficiently illustrated by the follow- 
ing table, which gives the distance, in millimeters, between the edges of 
the two pinne, each measured 20 centimeters below the tip, at intervals, 
for two days: 
Dee. 7. | 
Leaf. |- ] sar aa EE. od | 
7a.m. | a.m. | Qa.m. 10a.m. lLa.m, | 12m. Lp.m. | 
a A a | es jae = el a | 
A 20.3 | 20,3 | 17.5 | 15 18 12.5 12.5 | 
B 25 25 | 21 20 18 7 | 6.5 
| 
ao | co a oe = - - 
Dec. 7. | Dec. 8. | 
7 eames cae — | 
2p.m. 3.30p.m. 4.10p.m. | 6.30a.m. 7.308, m. 8.30 a.m. | 
pane en ” | 
A | 13 | 13 | 17 | 20.3 20.3 20 | 
B 16 | 17.5 21.5 25 | 25 | 28 
| 
—— ae = 
Dee. 8. | 
ae ue Dee. 9, 
Leaf. | 6.3) 
9.30a,m 4 12m. 1p.m./2p.m./4 p.m. Op. Mm: | > a 
nee eel ied 
| | | | | 
A 16.5) 13.5) 12} 18 | 125] 185] 165) 20.5 
B 21 1.5} 16| 165) 15 | 17 | 2 25 | 
As the accompanying records show,'* the months between November 
and April were exceedingly dry at San Ramon. The influence of this 
climatic condition on the behavior of the hinge is shown by the following 
measurements, in millimeters, made on the same leaves at the same points 
one month later than the date of the preceding table: 
| a | 
| | Jan. 11. | | 
ee _ Jan. 12, | Feb. 8, | 
tee | 7a.m. | 7a.m. 
6.30 a.m. 7.30 a, m, | 
| | 
ai — 
A |] 18 18 18 17 
B | 23.5 23.5 23 | 21 | 
The surface of leaf B was moist on the morning of January 11, and this 
fact demonstrated that its failure to open as widely as it did a month 
4 See hygrometric readings appended. 
