ON FRUITS AND SEEDS. 159 



nacea, the arrow-root, a West Indian species allied to Canna^ that if 

 the plant has had a severe shock it can not 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 fer- 

 tilized by bees, butterflies, and other day insects, sleep by night, if at 

 all ; while those which are dependent on moths rouse themselves tow- 

 ard 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 (X. cymhalaria) pushes 

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

 round and endeavors 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 the Potainogeton {Trapa natans). 

 In Vcdisneria, 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, b), 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 fertilization, how- 

 ever, 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 expe- 

 rience 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 local- 

 ities suitable to the requirements of the species. Thus a common 

 European sxf ecies, JCanthium spinosum, has rapidly spread over the 

 whole of South Africa, the seeds being carried in the wool of sheep. 

 From various considerations, however, it seems probable that in most 

 cases the provision does not contemplate a dispersion for more than a 

 short distance. 



There are a great many cases in which plants possess powers of 

 movement directed to the dissemination of the seed. Thus, in Geas- 



