52 DISPERSION OF SEEDS. [CHAP. 



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 

 desrrable 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 germi- 

 nating in new localities suitable to the requirements 

 of the species. Thus a common European species, 

 Xanthium 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 dissemina- 

 tion of the seed. 



I have already referred to the case of the common 

 Dandelion. Here the flower-stalk stands more or less 

 upright while the flower is expanded, a period which 

 generally lasts for three or four days. It then lowers 

 itself, and lies more or less horizontally and concealed 

 during the time the seeds are maturing, which in our 

 summers occupies about twelve days. It then again 

 rises, and, becoming almost erect, facilitates the 



