192 NINETEENTH REPORT. 



plant temperature and growth rate. Figure 8 shows the same growth 

 rate curve in connection with curves of soil and maximum and mean air 

 Temperature readings, all of which seem to be related, but none as closely 

 as that between tlie plant temperature and rate of growth. 



Another exi^eriment, which gave interesting results, was conducted as 

 follows: On April 6, 1916, before there was any visible sign of awaken- 

 ing in plant, life, and no apparent swelling of buds, four small fruit 

 trees, one each of peach, apple, pear and cherry, were dug up from the 

 college nursery and potted in large pots which were placed in the botany 

 department greenhouse. On the nineteenth day of April, thirteen days 

 after removal to the greenhouse, the cherry blossomed, but the others 

 failed to produce blossoms. The first blossom out of doors on the cherry 

 of the same variety came on May 9, thirty-three days after the date of 

 placing the trees in the greenhouse. Temperatures were recorded, both 

 out of doors and in the greenliouse, by means of thermographs, the green- 

 house temperature varying but little from 71° ¥. during the 13 days. 

 From these temperature traces the total number of "temperature-hours" 

 was computed, indoors for the 13 days and out-of-doors for the 33 days, 

 by giving to each hour a value equal to its temperature, minus 42°. This 

 gave for the thermal sum in the greenhouse 9018, and for the open air 

 4228. Evidently some other temperature than that of the air was active 

 in making up the difference, because, while other factors undoubtedly 

 entered into the problem, the question of heat was without doubt the 

 most vital one concerned. A third temperature trace was constructed by 

 interpolation from the four daily plant temperature readings, (minimum. 

 7 a. m., 2 p. m. and 7 p. m.), which was, of course, only roughly correct, 

 but which gave a total of 7877 units, by the process mentioned above. 

 The trees in the greenhouse were shaded from the sun and may have 

 been cooler than the bulb of the thermograph nearby, due to transpira- 

 tion. If we use Darwin's figures, already mentioned, and assume that 

 transpiration reduced the temperature of the trees by 1.5° C, (2.7° F), 

 then we would have for the sum of the thermal values in the greenhouse 

 8237, as compared with 7877 out-of-doors, so that the results obtained 

 by considering the teni[)eraturc of the plants themselves agree remark- 

 ably closely. 



It is a fact Ihal tlu; liigher temjierature produced by sunshine is only 

 one factor in promoting plant growth, as the actinical effects of sun- 

 light play an important part. The increase in temperature of the plant 

 when bathed in sunshine is probably in i)roportion to the actinical value 

 of sunlight, so that the temperature readings of tlic ))lant in sunshine 

 follow quite closely the resulting growth rate. 



