414 STEUCTUEE OF CHAP. VII. 



There is another movement, which since the time 

 of Linnaeus has generally been called sleep, namely, 

 that of the petals of the many flowers which close at 

 night. These movements have been ably investigated 

 by Pfeffer, who has shown (as was first observed by 

 Hofmeister) that they are caused or regulated more 

 by temperature than by the alternations of light and 

 darkness. Although they cannot fail to protect the 

 organs of reproduction from radiation at night, this 

 does not seem to be their chief function, but rather 

 the protection of the organs from cold winds, and 

 especially from rain, during the day. The latter 

 seems probable, as Kerner * has shown that a widely 

 different kind of movement, namely, the bending down 

 of the upper part of the peduncle, serves in many 

 cases the same end. The closure of the flowers will 

 also exclude nocturnal insects which may be ill-adapted 

 for their- fertilisation, and the well-adapted kinds at 

 periods when the temperature is not favourable for 

 fertilisation. Whether these movements of the petals 

 consist, as is probable^ of modified circumnutation we 

 do not know. 



Embryology of Leaves. A few facts have been in- 

 cidentally given in this chapter on what may be called 

 the embryology of leaves. With most plants the 

 first leaf- which is developed after the cotyledons, 

 resembles closely the leaves produced by the mature 

 plant, but this is not always the case. The first 

 leaves produced by some species of Drosera, for instance 

 by D. Capensis, differ widely in shape from those 

 borne by the mature plant, and resemble closely the 

 leaves of D. rotundifolia, as was shown to us by Prof. 

 Williamson of Manchester. The first true leaf of 



' Die Schutzmittel des Pollens,' 1873, pp. 30-39. 



