THE LARGER AQUATIC VEGETATION 179 



present in the embryo. The rigid segments of the forked leaves 

 frequently catch on the bottom so that a portion of the stem may 

 become buried and secure the plant to the soil. Just as often, 

 however, the plants float free in the water at the mercy of any 

 influence that may arise to change their relative position. Exam- 

 ination shows the entire surface of this plant to be so uniform in 

 structure that it makes no difference what part of the plant body 

 is vertical or horizontal in the water. 



Attachment, therefore, favors and necessitates differentiation 

 into specialized organs. 



In land plants the roots are organs of absorption as well as of 

 attachment, but until recently the general understanding has been 

 that the roots of aquatic plants serve for anchoring only. In- 

 vestigations of the writer have proved that the roots act as organs 

 for the absorption of mineral matter from the substratum and in 

 this respect are perfectly analogous to the roots of land plants. 



Root-hairs are present on the roots of terrestrial plants with but 

 comparatively few exceptions. These delicate structures are uni- 

 cellular with thin walls and are formed by the enlarging and pro- 

 truding of the ordinary peripheral cells of the root. Their presence 

 greatly increases the absorbing surface exposed to the soil and thus 

 the passage of mineral matter into the plants is provided for with 

 a minimum expenditure of tissue. Several authors have stated 

 that root-hairs are absent in the case of submerged aquatics. This 

 does not seem to be the case, however, as the writer has found them 

 present on 17 out of the 20 species common in Lake Erie. Even 

 without experimental evidence it would be reasonable to suppose 

 that the presence of root-hairs indicates that the roots act as organs 

 for the passage of mineral matter into the plant. Such delicate 

 structures can hardly be regarded as lingering rudiments of more 

 active organs present when perhaps the species was terrestrial. 



Land plants have developed a highly specialized tissue system 

 adapted to the transfer of water from the roots to stem, branches, 

 and leaves. This conductive tissue is usually called the vascular 

 system and the necessity for it in land plants is very apparent when 

 the rapidity with which water passes from the plant is taken into 

 account. That water plants likewise have conductive tissue has 



