ECOLOGICAL CLASSIFICATION OF -PLEANTS 29 
begin with, they are far more uniform than those to which a 
land-plant is exposed; there are not the sudden changes of 
temperature, which sometimes affect a land-plant so dis- 
-astrously, because water gets hot less quickly and loses its 
heat less quickly than land. The processes of nutrition are 
also more easily carried on, for water contains more carbon 
dioxide in proportion than air, and an increase of carbon dioxide ~ 
up to a certain limit makes assimilation more easy. Further, 
the whole surface of a water-plant is able to absorb water 
and substances dissolved in the water ; this explains the tact 
that water-plants often have not roots, and when present they 
serve rather as organs of attachment than of absorption. The 
ease with which nutrition is accomplished leads to rapid 
development and growth, and this results in extensive vegeta- 
tive reproduction ; any portion of a water-plant that is broken 
off can exist independently, as it is not dependent on the 
production of roots for its food-supply. This is well seen in 
the case of the American water-weed, Elodea canadiensis, which 
has spread in the course of sixty years through western 
Kurope, entirely by means of the breaking off of twigs, that 
have grown into new plants. In temperate regions, water- 
plants form special winter buds at the ends of their stems to 
protect themselves against frost ; these drop off and fall to 
the bottom of the pond, where they remain during the winter, 
then the following spring they come » and ee into 
vegetative shoots. 
The stems of water-plants have long sieriades and by the 
reduction, or almost entire absence of woody tissue, are very 
pliable, so that they are easily swayed by the current of water 
without breaking. 
The leaves of water-plants vary very much. Those that 
live in swamps or marshes, or along the banks of a river 
where the water is either stagnant or moving very slowly, 
have narrow elongated leaves—e.y., Iris, Loosestrife, Water 
Plantain, Bulrush. Leaves of this description are not so 
easily submerged, therefore the plant is not affected by 
floods. The leaves of plants which live entirely under water 
