418 



EFFECT OF LIGHT 



[Cn. XVII 



attached to the under side of rocks, hidden in sand or secreted 

 in other shady places. Insects affix their eggs to the under 

 side of leaves, provide nests for them, bury them in the earth, 

 in masses of food, in a hundred places hidden from direct sun- 

 light. Even the larvae, which swarm on the surface of seas and 



lakes, sink before the rising sun and 

 find protection beneath the superin- 

 cumbent strata of water.* From these 

 facts we may conclude that, in general, 

 growth does not take place in nature 

 in full exposure to sunlight. 



Diffuse daylight, even the light which 

 is essential to all healthy green plants, 

 markedly affects their growth. This 

 is a matter of every-day observation. 

 Who has not observed the contrast be- 

 tween the elongated, scraggy form of 

 plants grown in the dark and the re- 

 pressed, compact form characteristic of 

 the light? (Fig. 118.) Experiments 

 and comparative measurements give 

 us an insight into the degree of this 

 difference. Take, for example, the 

 case of tubers. SACHS ('63) planted 

 similar potato tubers in flower-pots. 

 One pot was covered with a clear bell- 

 jar and placed in the window ; the 

 other, which served as a control, was 

 covered by a large flower-pot, thus re- 

 maining in the dark. Both pots were 

 kept equally moist. At the end of a 

 period of 53 days, while the control 

 tubers had produced sprouts from 150 

 to 200 mm. high, those which had been 



FIG. 118. Two seedlings of 

 Sinapis alba of equal age. 

 E, reared in the dark, etio- 

 lated. N, reared in ordi- 

 nary daylight, normal. 

 Root hairs arising from 

 the roots. (From STRAS- 



BURGER, NOLL, SCHENCK, 



and SCHIMPER, Textbook 

 of Botany.) 



* The degree of protection from light afforded by layers of water is indicated 

 by certain calculations of WHIPPLE ('96), who finds that in a reservoir whose 

 color is slight (0.33, platinum standard), a layer of water one foot thick absorbs 

 25% of the light falling upon it, so that only 0.75 2 , or 56%, of the light at the sur- 

 face gets below two feet ; 0.75 8 , or 42%, below three feet, and so on. 



