AND OF LIGHT. 



245 



It should be pointed out that some animals, such as the 

 sable, musk-sheep, and raven, retain their dark colour 

 throughout the Arctic winter, so 

 that the reaction of the above-men- 

 tioned animals to cold, whether 

 direct or indirect, is a special and 

 not a general phenomenon. 



Light. The effect of light upon 

 growth, especially in plants, is 

 well known to be very considera- 

 ble. One might infer, therefore, 

 that differences in the intensity of 

 the light to which an organism is 

 subjected would form a potent 

 cause of variation. Such is actu- 

 ally the case among members of 

 the Vegetable Kingdom, though 

 only exceptionally so among those 

 of the Animal Kingdom. 



If plants be allowed to grow in 

 absolute darkness, they, as a rule, 

 become very much elongated in 

 form, whilst their leaves are small 

 and ill-shaped. The accompanying 

 figure shows the relative growth of 

 two seedlings of Sinapis alba of 

 the same age, one of them reared 

 in the dark ; and the other in ordi- 

 nary daylight.* Sachs found that potato tubers grown 

 in darkness for 53 days produced sprouts from 150 to 

 200 mm. high, whilst similar ones grown in day- 



* From Strasburger, Noll, Schenck, and Schimper'a " Textbook of 

 Botany." Quoted from Davenport's "Experimental Morphology," 

 p. 418. 



FIG. 23. Seedlings 

 of Sinapis alba. E, 

 reared in the dark. N, 

 reared in ordinary 

 daylight. 



