Chap. II. 



PULYINI OF COTYLEDONS. 



113 



cushion, or joint (as this organ has been variously 

 called), like that with which many leaves are provided. 

 It consists of a mass of small cells usually of a pale 

 colour from the absence of chlorophyll, and with its 

 outline more or less convex, as shown in the annexed 

 figure. In the case of Oxalis 

 sensitiva two-thirds of the 

 petiole, and in that of Mi- 

 mosa pudica, apparently the 

 whole of the short sub- 

 petioles of the leaflets have 

 been converted into pulvini. 

 With pulvinated leaves (i.e. 

 those provided with a pul- 

 vinus) their periodical move- 

 ments depend, according to 

 Pfeffer,* on the cells of the 

 pulvinus alternately expand- 

 ing more quickly on one side 

 than on the other; whereas 

 the similar movements of 

 leaves not provided with pul- 

 vini, dbpend on their growth 

 being alternately more rapid 

 on one side than on the 

 other.t As long as a leaf 

 provided with a pulvinus is 

 young and continues to grow, 

 its movement depends on both these causes combined ;| 

 and if the view now held by many botanists be sound, 

 namely, that growth is always preceded by the expan- 

 sion of the growing cells, then the difference between 

 the movements induced by the aid of pulvini and 



Oxalis rosea : longitudinal section 

 of a pulvinus on the summit 

 of the petiole of a cotyledon, 

 drawn with the camera luciJa, 

 magnified 75 times : p, p^ pe- 

 tiole ; /, fibro-vascular bundle ; 

 6, 6, commencement of blade ol 

 cotyledon. 



* 'Die Perioclische Bewegun- 

 gcn der Bliittorgane,' 1875. 



t Batalin, 'Flora,' Oct. 1st, 1873 

 t Pfeffer, ibid. p. 5. 



