202 BOOKS AND CURRENT LITERATURE 



culms at two-hour intervals during several days, and the growth rates 

 were compared with the data of relative humidity and air temperature 

 for corresponding periods. 



At Hakgala the humidity varied during the day, but was very close 

 to the saturation point throughout the night. During the day the 

 growth rates of the bamboo culms varied directly with changes in rela- 

 tive humidity, which ranged from 88 to 100 per cent, in spite of changes 

 in temperature, which ranged from 10°-21°C. The rates of growth 

 at night were usually much higher than the day rates. They gradu- 

 ally decreased from sunset until sunrise, thus varying directly with 

 the rather small changes in temperature from 17° to 13°C. Tempera- 

 ture also seemed to be effective in producing very rapid growth during 

 daylight periods of rain; since humidity was high at such times and the 

 higher diurnal temperatures produced more rapid growth than occurred 

 at night even with the same humidity. Thus, with nearly constant 

 humidity, during cloudy days and at night, temperature changes 

 appear to have been of primary importance in controlling growth. 



Within the same temperature range, but with larger variations in 

 humidity, the day rate was apparently mainly influenced by vari- 

 ations in humidity. Thus there were two limiting factors operating at 

 Hakgala, temperature and humidity. 



' At the other two stations the relations of growth rates to temper- 

 ature and humidity conditions were similar to those just described, 

 but the response of the plant to temperature changes were here less 

 marked than was the response to humidity alterations; changes in tem- 

 perature were here comparatively slight (from 22° to 27°C), but the 

 humidity varied markedly and frequently. 



The author emphasizes the point that relative humidity can be the 

 limiting factor only in an indirect way, the real limiting condition 

 being probably the transpiration rate, which is influenced by direct 

 solar radiation, wind movement and the degree of stomatal closure, as 

 well as by humidity. At all three stations the growth rates increased 

 very markedly at sunset and decreased as promptly at sunrise, but the 

 changes in humidity and temperature at these times were not considered 

 to be rapid enough to account fully for these sudden alterations in growth. 

 Smith attributes these alterations rather to the closure of the stomata 

 at sunset and their reopening at sunrise and to the changes thus brought 

 about in the rate of transpiration than to any direct influence of light 

 upon growth. 



Somewhat similar results were also obtained with a number of other 

 plants.— F. T. McLean. 



