212 The Air-vesicles of Bludderworts. 



at tlie period of fecundation. After this is completed the 

 utricle gradually fills with liquid; the specific gravity of the 

 plant increases, and it descends slowly with its fruit, sinking 

 "below the level of the water, and the seeds fall from their 

 capsule in the soil in which they are to germinate. We 

 find amongst authors a difference of opinion as to the position 

 of the Bladderworts in the water before flowering. Some regard 

 them as attached to the soil by slight roots ; others, like 

 Reinsch, consider them to be floating plants. They are at first 

 really attached to the soil at the bottom of the water; but 

 the air vesicles which develop on their leaves gently drag" 

 them out of the mud/ and in this action I see the true use 

 of the utricle, for the entire plant floats very well in the water, 

 and rises to the surface. 



I placed a tuft of Bladderwort while the vesicles were still 

 green in a large vessel of water, and found this to be the 

 case. The water snails in the same vessel eat up all the 

 vesicles, and the plant still floated. 



Bladderworts are not the only plants in which movements 

 are produced by disengagements of gas. In Hottoniu,Aldrovunda 

 and Trupu nutans, we observe at the flowering season slow 

 movements which displace the entire plant, while in other 

 aquatic species, such as Nympheu, Vallisneria, Ranunculus 

 aauatilis, etc., it is only certain parts which elongate themselves. 

 In the Bladderwort and Aldrovanda it is the air vesicles 

 which diminish the specific gravity, uproot it from the soil, 

 and cause it to ascend. In the Hottonia, air cells are found 

 amongst the leaflets, and in the petioles of Trapa nutans air 

 cavities are formed before inflorescence. 



Sometimes a plant cannot completely detach itself from 

 the soil, and the grains of pollen are then preserved from con- 

 tact with the water by another method, and one conspicuous 

 instance may be cited of an evolution of gas, which, instead of 

 moving the plant, plays a more direct part in the process of fecun- 

 dation. In the Lake of Escoubous, at the top of the High 

 Pyrenees, 2,052 metres above the sea-shore, a remarkable 

 variety of Ranunculus aauatilis grows, and form extensive 

 beds, anchored to the bottom of the water by rootlets, which 

 push their way among a thick carpet of dark green tremelloid 

 rdva. In this situation, contrary to the laws which determine 

 aquatic plants to seek the free air to accomplish their inflores- 

 cence and reproduction, it remains constantly submerged, far 

 from the banks, where the sharpness of the frosts might destroy 

 it, and far also from great depths, where it would not find 

 light enough for its growth. It spreads out its finely divided 

 leaves, and its white corollas, gilt at the bottom, and Ihe pro- 

 cesses of fecundation and reproduction takcplace without moving 



