CHAP. VI. SLEEP OF COTYLEDONS. 313 



grown older, has been acquired for the sake of pro- 

 tecting them from radiation at night ; but then we 

 should remember that there are many plants, the 

 leaves of which sleep, whilst the cotyledons do not ; 

 and if in some cases the leaves are protected from cold 

 at night whilst the cotyledons are not protected, so in 

 other cases it may be of more importance to the species 

 that the nearly full-grown cotyledons should be better 

 protected than the young ones. 



In all the species of Oxalis observed by us, the coty- 

 ledons are provided with pulvini ; but this organ has 

 become more or less rudimentary in 0. corniculata, 

 and the amount of upward movement of its cotyledons 

 at night is very variable, but is never enough to be 

 called sleep. We omitted to ascertain whether the 

 cotyledons of Geranium rotundifolium possess pulvini. 

 In the Leguminose all the cotyledons which sleep, as 

 far as we have seen, are provided with pulvini. But 

 with Lotus Jacobseus, these are not fully developed 

 during the first few days of the life of the seedling, 

 and the cotyledons do not then rise much at night. 

 With Trifolium strictum the blades of the cotyledons 

 rise at night by the aid of their pulvini; whilst the 

 petiole of one cotyledon twists half-round at the same 

 time, independently of its pulvinus. 



As a general rule, cotyledons which are provided 

 with pulvini continue to rise or sink at night during 

 a much longer period than those destitute of this organ. 

 In this latter case the movement no doubt depends on 

 alternately greater growth on the upper and lower side 

 of the petiole, or of the blade, or of both, preceded 

 probably by the increased turgescence of the growing 

 cells. Such movements generally last for a very 

 short period for instance, with Brassica and Grithago 

 for 4 or 5 nights, with Beta for 2 or 3, and with 



