Chap. IX. SENSITIVENESS TO LIGHT. '159 



excess of superfluous light, for the cotyledons became 

 but little and slowly curved even in the nearest pot. 

 Close to the 6th pot, at the distance of 20 feet from 

 the lamp, the light allowed us just to distinguish 

 a dot 3"56 mm. ('li inch) in diameter, made with 

 Indian ink on white paper, but not a dot 2'29 mm. 

 (•09 inch) in diameter. 



The degree of curvature of the cotyledons of Phalaria 

 within a given time, depends not merely on the 

 amount of lateral light which they may then receive, 

 but on that which they have previously received from 

 above and on all sides. Analogous facts have been 

 given with respect to the nyctitropic and periodic 

 movements of plants. Of two pots containing seedlings 

 of Phalaris which had germinated in darkness, one was 

 still kept in the dark, and the other was exposed (Sept. 

 26th) to the light in a greenhouse during a cloudy day 

 and on the following bright morning. On this morn- 

 ing (27th), at 10.30 a.m., both pots were placed in a 

 box, blackened within and open in front, before a 

 north-east window, protected by a linen and muslin 

 blind and by a towel, so that but little light was 

 admitted, though the sky was bright. Whenever the 

 pots were looked at, this was done as quickly as pos- 

 sible, and the cotyledons were then held transversely 

 with respect to the light, so that their curvature could 

 not have been thus increased or diminished. After 

 50 m. the seedlings which had previously been kept 

 in darkness, were perhaps, and after 70 m. were cer- 

 tainly, curved, though very slightly, towards tht 

 Avindow. After 85 m. some of the seedlings, which 

 had previously been illuminated, were perhaps a little 

 affected, and after 100 m. some of the younger ones 

 were certainly a little curved towards the light. At 

 this time (i.e. after 100 ni.^ there was a plain difference 



