404 MOVEMENTS. 



X 



would sooner or later be succeeded by its reversal ; and thus the 

 summit would be made to circumnutate. 



1049. Darwin's 1 illustration of the movements of the parts of 

 seedlings gives a clear idea of their sequence. " A man thrown 

 down on his hands and knees and at the same time to one side 

 by a load of hay falling on him, would first endeavor to get his 

 arched back upright, wriggling at the same time in all directions 

 to free himself a little from the surrounding pressure ; and this 

 may represent the combined effects of apogeotropism and cir- 

 cumnutation when a seed is so buried that the arched hypocotyl 

 or epicotyl protrudes at first in a horizontal or inclined plane. 

 The man, still wriggling, would then raise his arched back as 

 high as he could ; and this may represent the growth and con- 

 tinued circumnutation of an arched hypocotyl or epicotyl before 

 it has reached the surface of the ground. As soon as the man 

 felt himself at all free, he would raise the upper part of his bod}', 

 whilst still on his knees and still wriggling ; and this may repre- 

 sent the bowing backwards of the basal leg of the arch, which in 

 most cases aids in the withdrawal of the cotyledons from the 

 buried and ruptured seed-coats, and the subsequent straight- 

 ening of the whole hypocotyl or epicotyl, circumnutation still 

 continuing." 



1050. The cotyledons not only share the movement of the 

 caulicle, but they have also an independent movement which 

 is greatly modified by slight changes in the surroundings. Freed 

 from their seed-coats, they move upwards and downwards in very 

 harrow ellipses, and at different rates in different plants. Gen- 

 erally their movement takes place only once in the course of the 

 twenty-four hours : in Cassia tora, on an average, once in about 

 two hours ; in Oxalis rosea, once in about three hours ; while in 

 Ipomoea coerulea Darwin observed the change of position to occur 

 almost hourly. It is noticeable that the cotyledons ma}' change 

 the direction of their movement slightly at different times of the 

 da}', and may thus have a zigzag course during a part of the day 

 and a nearly regular orbit during the rest. 



1051 . In some of the seedlings which have been examined with 

 especial reference to their movements there is a joint or swelling 

 to be detected at the base of the petiole. This is the equivalent 

 of the pulvinus commonly found in Sensitive plants ; changes in 

 the position of cotyledons provided with such joints depend, as 

 in the case of sensitive leaves, upon variations in the turgescence 



1 Power of Movement in Plants, 1880, p. 106. 



