CHAP. IT. SENSITIVENESS OF COTYLEDONS. 125 



remained horizontal ; it was then brought back to the table, and 

 after 50m. the cotyledons had risen 68 above the horizon. 

 The other pot was placed during the same '2 h. behind a screen 

 in the room, where the light was very obscure, and the cotyledons 

 rose 63 above the horizon ; the pot was then replaced on the 

 table, and after 50 m. the cotyledons had fallen 33. Those two 

 pots with seedlings of the same age stood close together, and 

 were exposed to exactly the same amount of light, yet the coty- 

 ledons in the one pot were rising, whilst those in the other 

 pot were at the same time sinking. This fact illustrates in a 

 striking manner that their movements are not governed by the 

 actual amount, but by a change in the intensity or degree of 

 the light. A similar experiment was tried with two sets of seed- 

 lings, both exposed to a dull light, but different in degree, and 

 the result was the same. The movements of the cotyledons of this 

 Cassia are, however, determined (as in many other cases) largely 

 by habit or inheritance, independently of light; for seedlings 

 which had been moderately illuminated during the day, were 

 kept all night and on the following morning in complete dark- 

 ness; yet the cotyledons were partially open in the morning 

 and remained open in the dark for about 6 h. The cotyledons 

 in another pot, similarly treated on another occasion, were open 

 at 7 A.M. and remained open in the dark for 4 h. 30 m., after 

 which time they began to close. Yet these same seedlings, when 

 brought in the middle of the day from a moderately bright 

 into only a moderately dull light raised, as we have seen, their 

 cotyledons high above the horizon. 



Sensitiveness of Cotyledons to contact. This subject does not 

 possess much interest, as it is not known that sensitiveness of this 

 kind is of any service to seedling plants. We have observed cases 

 in only four genera, though we have vainly observed the coty- 

 ledons of many others. The genus Cassia seems to be pre-eminent 

 in this respect : thus, the cotyledons of 0. torn, when extended 

 horizontally, were both lightly tapped with a very thin twig for 

 8 m., and in the course of a few minutes they formed together 

 an angle of 90, so that each had risen 45. A single cotyledon 

 of another seedling was tapped in a like manner for 1 ra., and it 

 rose 27 in 9m. ; and after eight additional minutes it had risen 

 10 more ; the opposite cotyledon, which was not tapped, hardly 

 moved at all. The cotyledons in all these cases became hori- 

 zontal again in less than half an hour. The pulvinus is the most 

 (sensitive part, for on slightly pricking three cotyledons with a 



