INTRODUCTION II 
face for support. These, again, are of different habit 
according to whether the habitat is wet or dry. The 
division of water plants into entirely submerged, half- 
submerged, floating and littoral plants is connected 
with the derivation of land plants from water plants, 
since the latter were the forerunners of the former, 
though in one case the green algz of the fresh-water 
were derived from one or more types of seaweeds pure 
and simple. 
The trailing land plant is a water plant which has 
adopted a land existence. This may be shown from 
the manner in which aquatic plants may adopt an 
existence in a marsh where they are not wholly sub- 
merged, and finally, dry land conditions. There are 
many cases of land plants which have an aquatic 
variety, such as the Amphibious Knotweed, and the 
Buttercup Group has a batrachian section from 
which the land forms were derived. 
Then, again, there are the plants that have gonea 
step further than the trailing or prostrate habit, and | 
have adopted an erect but climbing habit. Not 
having accomplished the feat of becoming erect by 
unaided effort, they have to rely upon the support of 
other plants. 
It is unlikely that they were at first erect and have 
adopted the climbing habit. There is the case of 
the Ground Ivy, which is a trailing plant, and the 
Climbing Ivy which clings to trees. 
Many plants also have a prostrate form and an 
erect form, the latter probably derived from the first. 
