rERTILIZATION. 165 



one and then the other approaching the pistil and discharging 

 upon it the polliniferous contents of their anthers. 



When grains of pollen are thrown on water, the absorption 

 of the fluid is so rapid, that they burst, and a thick liquid 

 escapes from them which spreads itself over the surface of the 

 water. This thick liquid, in fig. 69, is seen escaping from one 



Tig. 69. 



of the pollen grains of Ipomoea hederacea, and is the fecundating 

 matter of the grain. The action of the pollen is therefore 

 liable to be frustrated by wet weather. This evil is guarded 

 against by the property which the anther-cells possess of open- 

 ing only in fine weather, as well as by the action of the floral 

 envelopes, which in some plants appear to be exceedingly 

 hygrometrical, enveloping the sexual organs on the slightest 

 appearance of any humidity in the atmosphere. The flowers 

 of the red ohiokweed (Anagallis) are a very remarkable illus- 

 tration of this phenomena. 



In this view too the economy of various aquatic plants is 

 exceedingly interesting, as for instance the pondweeds (Pota- 

 mogeton.) These plants live wholly submerged in the water; 

 but at the time of flowering, the peduncles or flower stalks 

 elongate so as to raise their flowers to the surface on which 

 they may be seen floating. The act of fertilization is thus 

 accomplished in the open air, and the ovaries are again drawn 

 beneath the water, where the seed ripens. The peduncles of 



15 



