MICHIGAN ACADEMY OF SCIENCE. 191 



A study of the tables giving the readings of the above thermometers, 

 reveals many interesting facts, only a few of which will be noted here, 

 and those mainly confined to the plant thermometer readings as com- 

 pared with the recorded air temperatures. In passing I would state, 

 however, that neither the soil nor solar radiation thermometer readings 

 seem to harmonize as closely with plant growth as do the readings 

 obtained in the plant itself. These relationships are shown in connection 

 with curves of growth rates, as reproduced in Figures 7 and 8. 



The plant thermometer readings were usually lower than the air 

 temperatures in the early morning, the minimum readings about 3° F. to 

 4° F. lower than the minimum temperatures recorded in the instrument 

 shelter. Differences were more pronounced, of course, when the weather 

 was clear, and the air still. The plant cooled off more rapidly in the 

 evening than the air which surrounded it, the 7 p. m. readings usually 

 registering 3° to 4° F. lower tlian the dry bulb thermometer. On very 

 warm days, with clear skies, a difference of 9° to 10° F. was registered 

 at 7 p. m., the plant cooling ofi" much faster than the air. But the most 

 striking difference in temperature occurred during the heat of the day, 

 frequently amounting to 20° F., and on a few days the plant thermometer 

 registered 36° ¥. higher than the air temperature, at the midday observa- 

 tion. On such dates the air was especially clear and still. On but 41 

 days out of the 304 on which observations were made at midday, did the 

 plant thermometer register lower tlian the air thermometer. These were 

 invariably dark and cloudy, many of them with rain falling at the time 

 of observation. 



It seems to me that these facts are significant and that plant tempera- 

 tures should be taken into account to a greater extent in any study of 

 the relation between temperature and plant growth. This may be 

 emphasized by noting the curves reproduced, herewith, in Figures 7 and 8. 

 The curve of plant growth was constructed from observations made of 

 the elongation growth in four plants, two Gladiolus gandavensis, and two 

 Glycine hispida (Soy bean), growing within a few feet of the thermome- 

 ters used in these observations. The measurements of growth v/ere made 

 by placing a marked leaf against a stake which had been firmly driven 

 into the ground alongside each plant at the start, and marking the height 

 of the leaf tip on the stake each day at 2 p. m. The total daily elonga- 

 tion of the four plants, in millimeters, was used in plotting the curves 

 of growth rate. The plants were kept well watered throughout the 

 experiment. In connection with the growth rate curve in Figure 7, there 

 is also charted a curve of plant temperatures and also one of the solar 

 thermometer readings. It will be noted that the parallelism between the 

 first two mentioned is marked, indicating a close relationship between the 



