1] EFFECT OF LIGHT UPON GROWTH 417 



sterilized and ordinary aquaria being quickly deprived of 

 living occupants. To the higher organisms, on the other hand, 

 sunlight is generally not fatal, but is usually unfavorable to 

 growth. This result is most clearly seen in seedlings. Thus 

 WIESNER ('79, p. 181) exposed seedlings of the vetch Vicia 

 sativa under a clear glass globe to sunlight, after having 

 marked off a centimeter's distance upon its zone of strongest 

 growth, and having placed it in a horizontal position so that it 

 should get the full force of the sun's rays. No growth occurred 

 during seven and a half hours, although the control, in a dark- 

 ened globe, turned its tip upwards and grew from 2.5 to 

 8.1 mm. during that period. A vertical seedling of the same 

 age was so protected by its foliage that in the sunlight it grew 

 from 0.5 to 1.2 mm. on the different sides. Thus, when the 

 growing part of a seedling is exposed to sunlight, little or no 

 growth occurs, and accordingly we find, as SACHS ('63) first 

 pointed out, that the growing tissue of vegetative points is 

 usually protected from the sun's rays. 



In animals, likewise, it is noteworthy that embryonic tissue, 

 and indeed the entire embryonic individual, is usually sheltered 

 from sunlight. In animals the embryo is sheltered in the dark- 

 ness of the maternal body ; in birds and reptiles the egg shells 

 are not merely mechanically resistant, but more or less opaque, 

 and, moreover, the whole egg is usually hidden from light.* 

 The delicate, often externally pigmented, embryos of Amphibia 

 are often buried by one of the parents or else develop among 

 weeds in the water. More rarely, as sometimes in the case of 

 frogs, they occur in open ponds, but then imbedded in a thick 

 envelope of albumen. Fishes usually bury their eggsf or affix 

 them to the under side of stones or place them in other shady 

 retreats. To the general rule, however, pelagic fish eggs seem 

 to constitute an important exception ; but some of these, per- 

 haps all, can change their level in the water (HEXSEX and 

 APSTEIX, '97, p. 63). Among molluscs, embryos are often, 

 retained in the shell of the parent or laid in capsules and then 



* BLANC ('92) has indeed shown that the development of the hen's egg is 

 much retarded when subjected in the incubator to daylight. 



t MILLET ('35) has shown that exclusion of light is the chief advantage 

 gained in this habit. 



2E 



