EXTERNAL INFLUENCES AS CAUSES OF VARIATION 245 



these limits protoplasm is stimulated by sudden changes, rapidly 

 becoming accustomed, however, to alterations within a narrow 

 range (phototonus), and soon resuming its normal activity, 

 except that as the intensity approaches the point of rigor activ- 

 ity appears to be permanently checked. 



Retarding effect of light upon the rate of growth. To the 

 higher organisms generally, sunlight, however intense, is not 

 fatal, but it not infrequently retards the rate of growth, espe- 

 cially among plants. This accounts for the relatively slower 

 growth of tropical vegetation as compared with that of higher 

 latitudes, and for the fact that growth in the sense of increase 

 in bulk is more rapid at night than in the daytime. Sachs 

 found 1 that the curve of growth reached its greatest height at 

 daylight, then commenced to decline, reaching its minimum a 

 little before sunset. Davenport points out that this fluctuation 

 is opposed to the effects of temperature, which is more favor- 

 able in the day than at night, so that the final results are some- 

 what less than the total influence due to light. 



This fact is well illustrated in experiments upon seedlings, 

 grown both in darkness and in light, investigations that are 

 entirely feasible, because at this stage the young plant depends 

 upon the old seed for its nourishment.' It is invariably found that 

 seedlings grown in the darkness have grown at the faster rate. 



That different rays have different effects upon the growth of 

 plants is easily shown. Flammarion cultivated sensitive plants 

 for three and a half months (July 4 to October 22) in red, green, 

 white, and blue light. At the close of the experiment the plants 

 had attained heights as follows : in the red light, 420 mm. ; in 

 the green, 152 mm. ; in the white, 100 mm. ; and in the blue, 

 27 mm., with general appearances shown in Fig. 27. 



In this experiment the greatest heat rays were of course trans- 

 mitted with the red light, but the general temperature was regu- 

 lated by currents of air passing through the various chambers. 2 



It has been roughly stated that light has no effect upon 

 germination. In general this is true, though careful experiments 

 indicate that most seeds germinate slightly earlier in darkness 



1 C. B. Davenport, Experimental Morphology, Part II, p. 421. 



2 Ibid. pp. 427-429. 



