WATER-PLANTS 
T 59 
as a rule, more or less far according to the presumable 
length of time the plants have been aquatics. In the case 
of the orders first mentioned, a study of the classificatory 
details will show how hard it is even to decide upon their 
approximate position in a natural scheme of classification. 
In the case of Ranunculus, Potamogeton, and others, the 
variability is enormous and leads to great difficulty in the 
determination of species, comparable to the cases of Rubus, 
Hieracium, &c. These plants having only recently com- 
menced their evolution into aquatics, it has not yet, perhaps, 
become complete. 
In general the conditions of water life are far more 
constant and uniform all over the world than those of 
existence on land. Accordingly the few species of sperma- 
phytic water-plants have a far wider distribution than land- 
plants. Most of the British forms are found throughout 
the north temperate zone. In the tropics other forms occur, 
which vegetate continuously the year round. In the tem- 
perate zones, on the other hand, hibernation is necessary, 
and may occur in a variety of ways. 
Ruppia, Zannichellia, Callitriche, Ceratophyllum and many others 
remain unaltered, though often sinking to the bottom of the water. 
Nymphaeaceae, Potamogeton natans , &c., hibernate in the form of 
rhizomes, stored with reserves to start growth in spring. Potamogeton 
pectinatus , Sagittaria, &c. form special tubers, something like the 
potato. A large number form special winter-buds — large buds at the 
ends of the stems, with a great number of closely packed green leaves 
provided with reserve food-stuffs. These usually drop off and spend 
the winter at the bottom of the pond, coming up and expanding in 
spring. Such are Utricularia, Hottonia, Myriophyllum, Hydrocharis, 
Potamogeton crispus and other species. More or less unique methods of 
hibernation occur in Lemnaceae, &c. (q-v.). 
Owing to the high specific and latent heat of water a 
water-plant is less exposed to violent changes of temperature 
than a land-plant, and if there be plenty of water is not 
liable to be interrupted in its growth by a drought in 
summer. These conditions favour its vegetative growth 
considerably, and it is still more aided by the peculiar 
conditions of nutrition. It takes in all or most of its food 
in solution from the water, which contains, owing to the 
difference in solubility of the two gases, more carbon dioxide 
and less oxygen in proportion than does the air. As the 
