70 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
the stems are in all cases well supplied with air spaces. Rootstocks are common 
and the plants generally spread locally by this means. 
ADAPTATION OF WATER PLANTS TO THEIR ENVIRONMENT. 
It is not the purpose of the writer to enter into the details of the structure of 
water plants, but a few general remarks indicating the relation between the structure 
of leaf and stem and the medium in which these plants grow will not be out of place. 
Aquatic plants show in form and structure special adaptations to their environ- 
ment. Their development is affected by dim light, the motion of the water, absence 
of transpiration, difficulty in obtaining oxygen, and the necessity of taking the min- 
eral substances needed through the entire plant system instead of by means of roots. 
Even in clear water there is some loss of light by filtration and by reflexion, and 
in the muddy water common in the Put-in Bai r region this loss must be considerable. 
Other things being equal, plants would lie limited in their growth by the depth of the 
water. That the plants within our region do not occupy all the places where depth of 
water would allow will be shown later on to be perhaps due to the character of the 
bottom. To make the most of the dim light that reaches them at the bottom of 
several feet of water, such plants as Earns and the Characea ? have numerous narrow 
leaves, always ascending and of the same structure on both sides (fig. H). The stem 
also is green and assists in the work of assimilation. A narrow or finely divided leaf 
is common among water plants. In TJtrimlaria , Ceratophyllum , Bidens beckii , and 
Ranunculus the leaf is split into many narrow divisions; in Elodea , JVaias, and in 
