598 . Sir John Luhhock [Feb. 18, 



In a gi'eat many cases leaves are said to sleejD ; that is to say, at 

 the approach of night they change their position, and sometimes fold 

 themselves up, thus presenting a smaller surface for radiation, and 

 being in consequence less exposed to cold. Mr. Darwin has proved 

 experimentally that leaves which were prevented from moving suffered 

 more from cold than those which were allowed to assume their natural 

 position. He has also observed with reference to one plant, Maranta 

 arundinacea, the Arrowroot, a West Indian species allied to Canna, 

 that if the plant has had a severe shock it cannot get to sleep for the 

 next two or three nights. 



The sleep of flowers is also probably a case of the same kind, 

 though, as I have elsewhere attempted to show, it has now, I believe, 

 special reference to the visits of insects; those flowers which are 

 fertilised by bees, butterflies, and other day insects, sleep by night, 

 if at all ; while those which are dependent on moths rouse themselves 

 towards evening, as already mentioned, and sleep by day. These 

 motions, indeed, have but an indirect reference to our present subject. 

 On the other hand, in the Dandelion [Leontodon), the flower-stalk is 

 upright while the flower is expanded, a period which lasts for three or 

 four days ; it then lowers itself and lies close to the ground for about 

 twelve days, while the fruits are ripening, and then rises again when 

 they are mature. In the Cyclamen the stalk curls itself up into a 

 beautiful spiral after the flower has faded. 



The flower of the little Linaria of our walls (L. cymhalaria) pushes 

 out into the light and sunshine, but as soon as it is fertilised it turns 

 round and endeavours to find some hole or cranny in which it may 

 remain safely ensconced until the seed is ripe. 



In some water plants the flower expands at the surface, but after it 

 is faded retreats again to the bottom. This is the case, for instance, 

 with the Water Lilies, some species of Potamogeton, Trapa natans, &c. 

 In Valisneria, again, the female flowers (Fig. 1 a) are borne on long 

 stalks, which reach to the surface of the water, on which the flowers 

 float. The male flowers (Fig. 1 6), on the contrary, have short, 

 straight stalks, from which, when mature, the pollen (Fig. 1 c) 

 detaches itself, rises to the surface, and, floating freely on it, is 

 wafted about, so that it comes in contact with the female flowers. 

 After fertilisation, however, the long stalk coils up spirally, and thus 

 carries the ovary down to the bottom, where the seeds can ripen in 

 greater safety. 



The next points to which I will direct your attention are the means 

 of dispersion possessed by many seeds. Farmers have found by 

 experience that it is not desirable to grow the same crop in the same 

 field year after year, because the soil becomes more or less exhausted. 

 In this respect, therefore, the powers of dispersion possessed by many 

 seeds are a great advantage to the species. Moreover, they are also 

 advantageous in giving the seed a chance of germinating in new 

 localities suitable to the requirements of the species. Thus a 

 common European species, Xanthium sjnnosumy has rapidly S2u-ead 



