44 



The Plant World. 



in the operation as could be desired. The psychrometer or wet 

 and dry bulb thermometer are more easily operated and are 

 quite satisfactory as regards operation, especially where the 

 humidities dealt with are not too low. I would call attention 

 here, however, to the inadequacy of the wet and dry bulb 

 thermometer without a strong current of air. The current 

 should be so strong that any increase in its velocity would pro- 

 duce no further lowering of the temperature of the wet bulb. 

 The hair hygrometer is unreliable unless often standardized by 

 some other instrument. Especially is this so in regions where 

 the humidity is usually low or where the fluctuations are very 

 great. Much improvement is possible in connection with this 

 factor. 



The evaporating power of the air, an integration of the tem- 

 perature, the wind velocity and the relative humidity, is at 

 least not as difficult of measurement as formerly. The porous 

 cup atmometer will give data for a curve as well as a final 

 integration for a long period. I think we may expect much 

 from this or some similar instrument. 



Precipitation data are readily obtained, with amply suffi- 

 cient accuracy, but the factors of superficial and subterranean 

 run-off as well as that of evaporation from the soil surface (all 

 of which are almost hopelessly difficult to measure) make these 

 data very hard to interpret, excepting for particular localities. 

 Their final interpretation will probably go hand in hand with 

 that of evaporation and soil moisture. 



For the measurement of light intensity — a factor which 

 has been shown conclusively to be of prime importance in many 

 ecological problems — we have at present no reliable and prac- 

 tical instrument. The delicate bolometer would doubtless 

 give the data needed, but it is not well suited to field work. 

 The so-called photometers (such as those used in photography) 

 are unsatisfactory in the extreme, both theoretically and prac- 

 tically. They are, of course, not photometers at all, but actino- 

 meters, and for our purpose it is unfortunate that the shorter 

 light waves are not the most important in plant activity. Here 

 again is a field that should prove wonderfully fertile to him who 

 has the courage and patience to cultivate it. 



The determination of the composition of the atmosphere 

 is important in certain lines of investigation and in the solution 



