Cucurbita maxima and G. Pejpo. 179 



the thermal range is from 25° to 30° 0., we shall find the growth 

 in the latter case to be greater as the following show, 

 where six observations covering the same hours of day are 

 compared. 



Aver, growth 

 No. of obs. Aver. temp. Total growth. per hour. Eel. hum. 



6 27-0° C 2-0 in. 0-333 in. high. 



6 34-9 1-6. " 0-266 " low. 



These figures show that there is a well defined limit to the 

 temperature which is most beneficial to plant growth ; and. 

 furthermore, that this limit is not fixed, but is variable ac- 

 cording to the attendant conditions. Certain exceptions to 

 the general law seem at times to appear, as when the most 

 rapid growth coincides with the maximum temperature, but 

 these instances are really conformable, and are capable of 

 explanation upon the ground that (1) the maximum tempera- 

 ture was in reality not very high, and (2) that if high, the 

 conditions of humidity were at the same time most favorable, 

 or (S) that these very rapid growths were only extreme fluctua- 

 tions in a very variable wave, and this will appear from a care- 

 ful inspection of the figures given. The same will be found 

 to hold true with reference to all the observations here recorded. 



The relative humidity of the atmosphere, or the degree of 

 saturation as dependent upon temperature, exerts a direct 

 influence upon the condition of tension in growing tissues, and 

 consequently upon the growth itself, by inducing more or less 

 rapid transpiration of moisture from the leaves. While, there- 

 fore, increasing temperature directly promotes growth on the 

 one hand, it indirectly retards it on the other. Again, taking 

 the figures already given, and comparing the atmospheric 

 humidity with the results there shown, it will be seen that 

 the greatest growth was obtained under conditions of greater 

 moisture, therefore when transpiration was reduced to the 

 minimum. On the other hand, the least growth was made 

 under conditions of less humidity, and therefore when transpi- 

 ration was active. Doubtless, also, the humidity of the soil 

 exerted an important influence upon the processes of growth 

 as observed, but, unfortunately, no record of this was kept, and 

 the relations which it bore to growth cannot now be brought 

 out as they should be. 



Alternations of day and night cause a marked influence 

 upon, and variation in, the phenomena of growth. Light gen- 

 erally exerts a retarding influence,* and other conditions being 

 equal, we should naturally expect to find the greatest elonga- 



* Sachs, Text Book, p. 755. Arbeiten das Bot. Inst, in Wurzburg, 1871. Bot. 

 Zeit, 1865. 



