CHAP. II. 



PULVINI OF COTYLEDONS. 



119 



When the cotyledons of 0. corniculata were dissected out of a 

 seed from which they would soon have naturally emerged, no 

 trace of a pulvinus could be detected ; and all the cells forming 

 the short petiole, 7 in number in a longitudinal row, were of nearly 

 equal size. In seedlings one or two days old, the pulvinus was 

 so indistinct that we thought at first that it did not exist ; but 

 in the middle of the petiole an ill-defined transverse zone of cells 

 could be seen, which were much shorter than those both above 

 and below, although of the same breadth with them. They 

 presented the appearance of having been just formed by the 

 transverse division of longer cells ; and there can be little doubt 

 that this had occurred, for the cells in the petiole which had 



Fig. 64. 



A. B. 



Oxalis corniculata : A and B the almost rudimentary pulvini of the coty- 

 ledons of two rather old seedlings, viewed as transparent objects. 

 Magnified 50 times. 



been dissected out of the seed averaged in length 7 divisions 

 of the micrometer (each division equalling '003 mm ), and were 

 a little longer than those forming a well-developed pulvinus, 

 which varied between 4 and 6 of these same divisions. After a 

 few additional days the ill-defined zone of cells becomes distinct, 

 and although it does not extend across the whole width of the 

 petiole, and although the cells are of a green colour from contain- 

 ing chlorophyll, yet they certainly constitute a pulvinus, which, 

 as we shall presently see, acts as one. These small cells were 

 arranged in longitudinal rows, and varied from 4 to 7 in number ; 

 and the cells themselves varied in length in different parts of the 



