CHAPTER V 



WATER-PLANTS 



AT the end of Chapter III. we pointed out that the 

 vegetation can be divided into two great series : (1) Those 

 which live in water, and (2) those which live on land. 

 The water-plant lives either within or upon liquid water, 

 and the conditions by which it is surrounded are conditions 

 that operate in water. The land-plant, on the other 

 hand, has its leaves and stems in the air ; it is therefore 

 exposed to the conditions that operate in air. The 

 distinction is fundamental. The submerged aquatic 

 differs from the land-plant in nearly all its vital relations 

 with the outside world. Light reaches it through the 

 water ; air comes to it from the water ; it cannot tran- 

 spire. The land-plant, as we have seen in the case of 

 xerophytes, is equipped for the perils of the land ; the 

 water-plant has to provide against the dangers that 

 threaten it through the water dangers arising from its 

 mode of life in water. How it meets them we shall see 

 when we have compared the characters exhibited by water- 

 plants with those exhibited by land-plants. 



Aquatics, or hydrophytes (Gr. hudor, water), may be 

 free-floating or anchored in the mud, and their leaves 

 and shoots may be floating or submerged. 



The Characters of Aquatic Plants compared with those 

 of Terrestrial Plants. 



1. Aquatics show an enormous increase in the amount 

 of internal air-space (Fig. 17). So abundant is this in some 

 organs, that the tissue is almost limited to the thin parti- 

 tion-walls which separate the air-chambers. The large 

 air-spaces in the leaves are continuous with the air- 



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