4O2 LECTURE XVI. 



It will also be observed that the maxima are not attained 

 during the periods of darkness, but shortly after the exposure 

 of the plant to light, and, further, that the occurrence of the 

 minima does not coincide in point of time with the greatest 

 intensity of light, but that they occur in the evening when the 

 intensity of the light is diminishing. These facts illustrate the 

 statements made above that the influence of light makes 

 itself only gradually apparent, and that it persists for a time 

 after the growing organ has been more or less completely with- 

 drawn from exposure to light. A comparison of the two 

 curves shews that the one (3 h) is more even than the other 

 (i h). This is due to the fact that when measurements of 

 growing organs are made at short intervals, the spontaneous 

 irregular variations (see p. 358) of the rate of growth become 

 much more apparent than when the measurements are made 

 at longer intervals. 



It must, however, be remarked that the alternation of light 

 and darkness is accompanied by other changes in the external 

 conditions, and that it affects various processes of the plant. 

 If we observe the curve of temperature in Fig. 45, for in- 

 stance, we see that it rises during the periods of exposure to 

 light, and falls during the periods of darkness. We know that 

 at a high temperature the rapidity of growth is greater, 

 whereas at a low temperature it is less ; hence, the different 

 prevailing temperatures constitute an important factor in 

 determining the form of the curve of growth. Again, a high 

 temperature and exposure to light promotes transpiration, 

 and we know that very active transpiration tends to retard 

 growth. The curves in the figure are therefore not merely 

 simply the expression of the action of light upon growth, but 

 they are the resultant expression of this action, of the influence 

 of various temperatures, and possibly of the influence of 

 variations in the activity of transpiration. The fall of the 

 curve of growth during a period of exposure to light is not 

 so rapid as it might be, because the retarding effect of light 

 is to some extent counteracted by the accelerating influence 

 of the higher temperature ; but, on the other hand, the 

 retarding effect of light may be assisted by the increased 



