260 LECTURES TO SCIENCE TEACHERS. 



where the tracing indicates a short epoch of growth. From 

 10 A.M. to 4 P.M. the influence of a high temperature seems 

 to be entirely overcome by the action of an intense light, 

 so that it is not until the afternoon, when the light begins to 

 wane, that the growth is resumed. 



Hitherto we have regarded only Figs. 4 and 5, which 

 indicate the actual absolute growth of the plant. Let us 

 briefly consider Fig. 6, which shows the relative velocity of 

 growth at different times. The growth was slowest at 

 4 P.M. at 8 P.M. the figure indicates that its rapidity was 

 twice as great then we observe a slight diminution of 

 velocity towards midnight, but an increase soon followed 

 and continued until 10 A.M., when the greatest velocity 

 three times that at 4 P.M. was attained. 



From the discussion of an experiment performed with 

 this apparatus we have derived a confirmation of the 

 general law which expresses the relations between plant- 

 life and temperature, and we have learned that light has a 

 retarding effect upon the process of growth. It may be 

 objected, however, that the intermissions of growth recorded 

 in the tracings may not, after all, be entirely due to the 

 influence of external conditions upon the plant, but that 

 they may perhaps be referred to some inherent property of 

 the growing tissues by virtue of which a certain periodicity 

 in the process is produced. Experiments have been in- 

 stituted in which the external conditions were prevented, 

 as far as possible, from undergoing any variation during 

 their performance, and in these no such intermissions of 

 growth were observed as are indicated in Figs. 4 and 5. 



We may now go on to inquire more deeply into the 

 action of light upon growth, and the first problem which 

 presents itself is to find what rays of the solar spectrum 

 are particularly concerned in it. We might obtain the 

 solution of this problem by exposing the growing parts of 

 a plant successively to the action of the rays of different 

 refrangibility which make up the solar spectrum, and by 

 comparing the rates of growth observed. We should find 

 that the rapidity of growth would gradually diminish as 

 the plant was removed from the red towards the blue end 

 of the spectrum, until it entirely ceased. This mode of 

 experiment is, however, scarcely suitable for the general 

 purposes of demonstration, as it must be carried out with 



