400 BOTANICAL GAZETTE [JUNE 
the turgidity of the cells in the petiole during the day, and it 
resumes its work, after a period of rest, long before there seems 
to be any direct stimulus from the light. 
EFFECT OF WIND, CLOUDS, DRYNESS, AND MOISTURE. 
A moderate wind has little or no effect on nutation, no dif- 
ference from what direction it comes. In Kansas the wind is 
very variable, but no important effect could be detected. How- 
ever, when the wind blows very strongly it interferes somewhat 
with nutation. On some days when there was a very strong 
southwest wind the plants still turned west from 70° to gO’. 
Cloudy weather has little effect on nutation. However, on 
some days it seems to check it a little. On some days, as for 
instance on July 27, 1897, it was cloudy nearly all the afternoon, 
but there was a strong nutation of 90° to the west. 
Continued drought has a marked effect on nutation. When 
the ground is dry and the weather clear and hot the plants wilt, 
the leaves droop, and there is little or no nutation of the stem. 
During rainy weather on cloudy days the nutation in the 
evening was scarcely noticeable. Also ona rainy day with sun- 
shine in the latter part of the afternoon little or no nutation was 
discovered. When the ground was saturated with moisture and 
the atmosphere was very humid, although clear, very little nuta- 
tion was observed. Thus in rainy weather, nutation is checke 
when it cannot be ascribed to a lack of sunshine. This was 
especially noticeable in the evening, but it was also very marked 
on mornings when the ground and atmosphere were saturated. 
When the ground is moderately moist and the air clear and dry 
with a light breeze, the conditions for nutation are at the opti- 
mum. At such times it is simply astonishing to notice a sun- 
flower patch just at the setting of the sun. It thus appears we 
excessive moisture interferes greatly with the normal nutation © 
Helianthus annuus. This agrees with the observations of — 
ningham,} who finds that plants react least readily to stimuli 10 
a saturated atmosphere. : 
3 The causes of fluctuations in turgescence in the motor organs of leaves. Ann- 
Roy. Bot. Gar. Calcutta 6 [Part 1, qto. pp. 161]. 1895. 
