OF VITAL PHENOMENA 155 



The geotropism of roots has been supposed to be due to the 

 action of starch grains in the cells as statoliths. According to 

 Czapek (1902) homogentisinic acid is produced in geotropic root 

 tips and phototropic sprout tips, but the mechanism of its action 

 is not clear. 



Loeb has shown that phototropism obeys the Bunsen-Roscoe 

 (1862) law. That is to say, the time required to induce a re- 

 action is inversely proportional to the intensity of the light. 



The perception of light is perhaps beyond the pale of physical 

 chemistry at present. It should be noted, however, that many 

 of the devices used in photography were first used in the con- 

 struction of the eyes of animals. Even the "Lttmiere" plate for 

 color photography is represented by the retina of turtles and 

 diurnal birds, which is covered with red and orange oil droplets. 

 These are absent in the eyes of nocturnal birds (Winterstein, 

 1910 iv 577). The rods and cones beneath a red oil drop can 

 be affected by red rays only and those beneath a yellow drop 

 by yellow rays only, whereas those not covered by oil drops 

 would be affected chiefly by the more effective blue rays. In 

 this way a mechanism for color vision is provided, but whether 

 it functions as such has not been determined. 



One striking characteristic of the behavior of organisms and 

 parts of organisms is their periodicity. The human body shows 

 a diurnal periodicity that is perhaps most strikingly demonstrated 

 in fluctuations in temperature, being coldest some time near 

 3 A. M. and warmest about 10 A. M. Each organ has its char- 

 acteristic periodicity, respiratory center 16 and heart 72 per 

 minute and the motor ganglia 50 per second. It is not always 

 easy to determine what factors determine the period when it is 

 very long. The Atlantic Palolo worm (Eunice fucata) comes 

 out of its hole to liberate the genital segments before dawn on 

 the last quarter of the moon between June 20 and July 25. The 

 light or tide of the last quarter are unnecessary (A. G. Mayer). 

 It is possibly the light of the full moon that prevents it from 

 swarming too soon (some swarmed near full moon this year 

 but another swarm occurred on the last quarter; it was not ob- 

 served whether the full moon was overcast). 



According to Bohn (1914) snails taken far from the sea- 

 shore maintain a rhythm synchronous with the tides for a con- 



