CHAT. IX. SENSITIVENESS TO LIGHT. 4f>5 



it was impossible to judge, excepting by the shadow 

 cast by an upright pencil on a white card, on which 

 side most light entered, so that the excess on one side 

 must have been extremely small. 



A pot with seedlings of Phalaris Canariensis, which 

 had been raised in darkness, was placed in a com- 

 pletely darkened room, at 12 feet from a very small 

 lamp. After 3 h. the cotyledons were doubtfully 

 curved towards the light, and after 7 h. 40 m. from 

 the first exposure, they were all plainly, though 

 slightly, curved towards the lamp. Now, at this dis- 

 tance of 12 feet, the light was so obscure that we could 

 not see the seedlings themselves, nor read the large 

 lioman figures on the white face of a watch, nor see a 

 pencil line on paper, but could just distinguish a line 

 made with Indian ink. It is a more surprising fact 

 that no visible shadow was cast by a pencil held 

 upright on a white card; the seedlings, therefore, 

 were acted on by a difference in the illumination of 

 their two sides, which the human eye could not dis- 

 tinguish. On another occasion even a less degree of 

 light acted, for some cotyledons of Phalaris became 

 slightly curved towards the same lamp at a distance 

 of 20 feet ; at this distance we could" not see a cir- 

 cular dot 2-29 mm. ('09 inch) in diameter made with 

 Indian ink on white paper, though we could just see a 

 dot 3-56 mm. (-14 inch) in diameter; yet a dot of 

 the former size appears large when seen in the light.* 



We next tried how small a beam of light would act ; 

 us this bears on light serving as a guide to seedlings 

 whilst they emerge through fissured or encumbered 

 ground. A pot with seedlings of Phalaris was cove-ed 



* Straslmrger says (' Wirluing Hicmatococcus moved to a light 

 de* Lichtes auf Sch \varmsporcn,' which only just sufficed to allovr 

 187S, p. 52), that the spores of middle-sized type to be read. 



CO 



