Chap. II. CIKCUMXU'J'ATION OF COTYLEDOXS. 109 



it is effected by the contraction of the parenchyma- 

 cells of the root. But the hypocotyl itself in some 

 cases contracts greatly, and although at first smooth 

 becomes covered with zigzag ridges, as we observed 

 with Giihago segetum. How much of the drawing 

 down and burying of the hypocotyl of Opuntia hadlaris 

 was due to the contraction of this part and how much 

 to that of the radicle, we did not observe. 



Oircumnutation of Cotyledons. — With all the dico- 

 tyledonous seedlings described in the last chapter, the 

 cotyledons were in constant movement, chiefly in a ver- 

 tical plane, and commonly once up and once down in 

 the course of the 24 hours. But there were many excep- 

 tions to such simplicity of movement ; thus the cotyle- 

 dons of Ipomoea casrulea moved 13 times either upwards 

 or downwards in the course of 16 h. 18 m. Those of 

 Oxalis rosea moved in the same manner 7 times in the 

 course of 24 h. ; and those of Cassia tora described 5 

 irregular ellipses in 9 h. The cotyledons of some 

 individuals of Mimosa pudica and of Lotus Jacdbseus 

 moved only once up and down in 24 h., whilst those of 

 others performed within the same period an additional 

 small oscillation. Thus with different species, and 

 with different individuals of the same species, there 

 were many gradations from a single diurnal move- 

 ment to oscillations as complex as those of the 

 Ipomcea and Cassia. The opposite cotyledons on the 

 same seedling move to a certain extent independently 

 of one another. This was conspicuous with those of 

 Oxalis sensitiva, in which one cotyledon might be 

 seen during the daytime rising up until it stood 

 vertically, whilst the opposite one was sinking down. 



Although the movements of cotyledons were gene- 

 rally in nearly the same vertical plane, yet their 

 upward and downward courses never exactly coin- 



