REPRODUCTION OF AQUATIC PLANTS 



49 



plants ; it is therefore considered that these two modifications 

 are the first stage towards the transformation of a terrestrial 

 plant into an aquatic plant. It is clear from the observations on 

 the Cardamine pratensis that these changes were brought about 

 by the influence of external conditions ; but this plant does not 

 appear able to hold its own as an aquatic plant, probably the 

 changes thus brought about by the pressure of environment are 

 not handed down, in the case of this plant at any rate, by natural 

 selection. 



THE REPRODUCTION AND DISPERSION OF AQUATIC PLANTS 



Life in the medium of water is favourable to vegetative re- 

 production. The rapid spread of the Elodea canadensis has 

 already been noticed (p. 22), the growth of the Frog-Bit, a closely 

 allied genus, may be given here. 

 This plant sends out long runners, 

 which give off at intervals tufts of 

 floating leaves above, and roots 

 hanging down in the water. This 

 method of multiplication is similar 

 to that of the Strawberry. In the 

 autumn, buds form at the end of 

 the shoots, sink to the bottom while 

 the rest of the plant decays, and the 

 following spring they rise to the 

 surface and grow into new plants. 

 The distribution of this plant is not 

 as wide as that of the Elodea, but 

 it does belong to both Europe and 

 Asia and is common in many parts 

 of England and Ireland. 



The Algae are usually fertilised 

 in water, but Flowering Plants are 

 not as a rule. The Brown and Red 

 Sea-weeds, the Green Algae of ponds are all reproduced, not 

 only by vegetative processes, but by fertilisation. In the 

 case of Flowering Plants, however, seed is not usually formed 



VOL. V. 4 



FIG. 19. Frogbit (Hydrocharis 

 M or sus- Ranee). 



