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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