150 THE GARDEN OF EARTH 



Perhaps the only case known of Cross-Fertilisation 

 being carried on by a warm-blooded animal, not an insect, 

 is that of Humming-birds. 



Ill — Curious Contrivances 



Thus it becomes clear that, in the various plans found 

 in Nature for the carrying of pollen from stamens to 

 pistils, a good deal may be seen of hindering and of 

 helping : of hindering the pollen from getting to the 

 wrong pistils ; of helping it to get to the right ones. That 

 a flower should be fertilised by its own pollen is bad ; 

 therefore hindrances are placed in the way. That the 

 pistils of one plant should be supplied with pollen from 

 other similar plants is to be desired ; therefore helps are 

 provided. 



A certain plant belonging to the south of Europe 

 grows abundantly in ponds or in shallow waters of a 

 lake ; and with it may be markedly seen both the hinder- 

 ing and the helping. Like many others, this — ^the 

 Vallisneria spiralis — has two kinds of flowers, the pistil- 

 bearing and the stamen-bearing. Its pollen is of a very 

 sticky nature. 



In mud at the bottom it has its roots; and long 

 slender leaves grow upward, still under water. There 

 too, the flowers quietly take shape; both kinds, pistil- 

 flowers and stamen-flowers, being sheltered safely inside 

 sacs or bladders. Each pistil-flower is alone within its 

 sac ; and on a lengthening stem it rises and rises till 

 the surface is reached. Then it opens out, with large 

 petals and wide-tipped hanging pistils, to wait for the 

 pollen which has to be brought to it. 



