CHAP. IX. SENSITIVENESS TO LIGHT. 461 



degree to which they have been previously illu- 

 minated. We shall presently see that the influence 

 of light on their bending continues for a short time 

 after the light has been extinguished. These facts, as 

 well as that of the curvature not increasing or de- 

 creasing in nearly the same ratio with that of the 

 amount of light which they receive, as shown in the 

 trials with the plants before the lamp, all indicate 

 that light acts on them as a stimulus, in somewhat 

 the same manner as on the nervous system of animals, 

 and not in a direct manner on the cells or cell- walls 

 which by their contraction or expansion cause the 

 curvature. 



It has already been incidentally shown how slowly 

 the cotyledons of Phalaris bend towards a very dim 

 light ; but when they were placed before a bright 

 paraffin lamp their tips were all curved rectangularly 

 towards it in 2 h. 20 m. The hypocotyls of Solanum 

 lycopersicum had bent in the morning at right angles 

 to wards a north -east window. At 1 P.M. (Oct. 21st) the 

 pot was turned round, so that the seedlings now pointed 

 from the light, but by .5 P.M. they had reversed their 

 curvature and again pointed to the light. They had 

 thus passed through 180 in 4 h., having in the 

 morning previously passed through about 90. But the 

 reversal of the first half of the curvature will have 

 been aided by apogeotropism. Similar cases were 

 observed with other seedlings, for instance, with those 

 of Sinapis alba. 



We attempted to ascertain in how short a time 

 light acted on the cotyledons of Phalaris, but this 

 was difficult on account of their rapid circumnutating 

 movement ; moreover, they differ much in sensibility, 

 according to age ; nevertheless, some of our observa- 

 tions are worth giving. Pots with seedlings were 



