CHAP. VIII. 



HELIOTROPISM. 



431 



Fig. 177. 



which, whilst young, is so extremely sensitive to light, woith 

 giving. 



Cassia tora. The cotyledons of this plant are extremely 

 sensitive to light, whilst the 

 hypocotyls are much less 

 sensitive than those of most 

 other seedlings, as we had 

 often observed with surprise. 

 It seemed therefore worth 

 while to trace their move- 

 ments. They were exposed 

 to a lateral light before a 

 north-east window, which 

 was at first covered merely 

 by a muslin blind, but as 

 the sky grew brighter about 

 11 A.M., an additional linen 

 blind was suspended. After 

 4 P.M. one blind and then the 

 other was removed. The 

 seedlings were protected on 

 each side and above, but were 

 open to the diffused light 

 of the room in the rear. Up- 

 right filaments were fixed to 

 the hypocotyls of two seed- 

 lings, which stood vertically 

 in the morning. The accom- 

 panying figure (Fig. 177) 

 shows the course pursued by 

 one of them during two days ; 

 but it should be particularly 

 noticed that during the 

 second day the seedlings were 



kept in darkness, and they Cas * ia tora -\ heliotropic movement and 

 then circumnutated round 

 nearly the same small space. 

 On the first day (Oct. 7th) 

 the hypocotyl moved from 

 8 A.M. to 12.23 P.M., toward 

 the light in a zigzag line, then turned abruptly to the left 

 and afterwards described a small ellipse. Another irregular 



circumnutation of a hypocotyl. 

 inch in height) traced on a horizontal 

 glass from 8 A.M. to 10.10 P.M. Oct. 

 7th. Also its circumnutation in 

 darkness from 7 A.M. Oct. 8th to 7.45 

 A.M Oct. 9th. 



