22 THE BOOK OF NATURE STUDY 



SUBMERGED AQUATIC PLANTS. Some aquatic plants live 

 an entirely submerged existence : they even flower, and are 

 pollinated under water ; they form seed, which is dispersed by 

 water. The habit of life of such plants is naturally very different 

 from that of land plants, and also from those aquatic plants 

 which have their roots and some of their leaves in water, but 

 otherwise lead a terrestrial existence. In some ways life is easier 

 for these plants. To begin with, they are not exposed to the 

 rapid changes of temperature that are often so trying to plants 

 that live under atmospheric conditions. The temperature of 

 water is more uniform ; even when a hard frost occurs, the sub- 

 merged vegetation is protected, for the ice floats to the top and 

 the plants are sheltered in the less cold water flowing beneath. 

 Again, they are not exposed to the frosts of early spring, which 

 affect the blossoms of land plants. Then the whole surface of a 

 submerged plant is able to absorb water and the substances 

 dissolved in the water, and this facilitates nutrition. Plants that 

 live an entirely submerged existence can only do so when the 

 water is sufficiently clear and the upper surface free enough of 

 vegetation to allow the rays of light to penetrate. The Alga, 

 Nitella, for instance, is found in Lake Constance at a depth of 

 about thirty yards ; in more turbid waters it is only found at a 

 depth of twelve yards. Below a depth of six yards it is unusual 

 to find Flowering Plants. Some of these submerged plants have 

 more or less upright stems with cylindrical leaves, as the Quillwort 

 and the Pillwort, found on the edges of lakes in clear water ; 

 others have their stems parallel to the surface of the water ; they 

 have long shoots, adapted for floating in the water, e.g., certain 

 species of Pondweed (Potamogeton). 



Submerged plants are usually very rapidly propagated. Many 

 do not form seed, but multiply vegetatively. A twig of a water 

 plant, broken off from the parent stem, is able to lead an inde- 

 pendent existence and form a new plant. It is in this way that 

 the American water-weed, Elodea canadensis, has spread in the short 

 space of sixty years through Western Europe ; it was first observed 

 in Britain in 1847, in Yorkshire, Leicestershire, and near Berwick 

 and Edinburgh ; now it is found in most ponds, canals, and slow- 

 flowing streams. 



