APPARATUS OF VEGETABLE PHYSIOLOGY. 259 



any rate would proceed but slowly, from 6 P.M. to 3 A.M., 

 and that then it would go on with increasing rapidity until 

 the afternoon. We find, however, as a matter of fact, 

 that growth has gone on vigorously from 6 P.M. to 3 A.M., 

 and that the tracings support our supposition only in 

 regard to the growth which took place between 7 A.M. and 

 10 A.M. We are forced to the conclusion that the growth 

 of the plant has been influenced by some force having an 

 effect upon it contrary to that of temperature and we 

 must seek this force in the action of light. 



Let us employ a process of reasoning in the investiga- 

 tion of this question, similar to that which we employed 

 in the case of temperature. We may begin by assuming 

 that which already appears to be probable, namely, that 

 the effect of light is to retard growth, and we may go on 

 to see whether or not this hypothesis satisfactorily explains 

 the phenomena. We should expect to find that with dim 

 light (from 6 P.M. to 3 A.M.) the process of growth would 

 commence, or, if already in progress, would go on with 

 increased rapidity ; and we should further expect to find 

 that, after sunrise, the rapidity of growth would dimmish 

 until it was completely arrested. The tracings bear out 

 the first of our expectations with precision, but the epoch 

 of growth extending from 7 A.M. to 10 A.M. is quite 

 contrary to the second. It is evident from these facts 

 that the intermissions of growth are not due to the action of 

 light alone, nor to the action of temperature alone : it 

 remains for us to see whether they are not due to the 

 combined action of these influences. From 7 P.M. to 3 A.M. 

 we have a dim light and a decreasing temperature. Of 

 these conditions, the former is favourable, the latter un- 

 favourable, to the process of growth. If we reflect that the 

 diminution of temperature must be after all comparatively 

 small, we may fairly conclude that the absence of light has 

 had a greater effect than the diminution of temperature, and 

 that therefore the process of growth has on the whole been 

 favoured. From 3 A.M. to 7 A.M. we have a rapidly increasing 

 intensity of light and a slowly rising temperature, and we 

 find, as we might have expected, that the tracing indicates 

 a period of inactivity. From 7 A.M, to 10 A.M. we have 

 again an increasing intensity of light and a rising temper- 

 ature, the latter taking place more rapidly than the former, 



