118 PULVINI OF COTYLEDONS. CHAP. IL 



the species seen by us are pulvinated ; so it is with 

 the cotyledons of T. svMerraneum and strictum, which 

 stand vertically at night ; whereas those of T. resupi- 

 natum exhibit not a trace of a pulvinus, nor of any 

 nocturnal movement. This was ascertained by mea- 

 suring the distance between the tips of the cotyledons 

 of four seedlings at mid-day and at night. In this 

 species, however, as in the others, the first-formed leaf, 

 which is simple or not trifoliate, rises up and sleeps 

 like the terminal leaflet on a mature plant. 



In another natural genus, Oxalis, the cotyledons of 

 0. Valdiviana, rosea, floribunda, articulata, and sensitiva 

 are pulvinated, and all move at night into an upward 

 or downward vertical position. In these several species 

 the pulvinus is seated close to the blade of the coty- 

 ledon, as is the usual rule with most plants. Oxalis cor- 

 niculata (var. Atro-purpurea) differs in several respects ; 

 the cotyledons rise at night to a very variable amount, 

 rarely more than 45; and in one lot of seedlings 

 (purchased under the name of 0. tropasoloides, but 

 certainly .belonging to the above variety) they rose 

 only from 5 to 15 above the horizon. The pulvinus 

 is developed imperfectly and to an extremely variable 

 degree, so that apparently it is tending towards abor- 

 tion. No such case has hitherto, we believe, been 

 described. It is coloured green from its cells con- 

 taining chlorophyll; and it is seated nearly in the 

 middle of the petiole, instead of at the upper end as 

 in all the other species. The nocturnal movement is 

 effected partly by its aid, and partly by the growth of 

 the upper part of the petiole as in the case of plants 

 destitute of a pulvinus. From these several reasons 

 and from our having t partially traced the develop- 

 ment of the pulvinus from an early age, the case 

 seems worth describing in some detail. 



