THE LARGER AQUATIC VEGETATION 199 



that the plant is unable to supply. In the other case the land 

 plant may persist and gradually reduce in complexity structures 

 already present. Thus it is that the reduction of the vascular 

 system has proceeded in the same manner in both the dicotyle- 

 dons and monocotyledons, so that the simplest stem structure is 

 alike for members of either group. In Ceratophyllum the vascular 

 system has become so simplified that its dicotyledonous relation- 

 ship cannot be estabHshed by the stem structure. 



How is the presence of stomata on submerged leaves to be ac- 

 counted for? It can hardly be supposed that they are developing 

 in anticipation of the time when the species is to have a transpiring 

 surface. More hkely is it that these stomata are reminders of 

 the time when the species had an exposed surface. 



In the genus Utricularia there are land as well as water species 

 and the presence of bladders characterizes both varieties. It is 

 hardly probable that submerged plants accustomed to the food 

 supplied in solution by the surrounding water should acquire an 

 appetite for animal food and develop such elaborate and imique 

 organs for securing nitrogenous nourishment. 



In some cases, as in Lemna, the ancestry is not so obvious and 

 convincing evidence is difficult to obtain. 



The pollination of Zannichellia under water has been previously 

 mentioned (Fig. 267). In such cases the evolution of stamens and 

 pistils cannot be regarded as having occurred in wholly submerged 

 plants. Much less difficult it is to understand how land plants 

 with stamens and pistils already developed could gradually be- 

 come adapted to an aquatic habit before these organs would dis- 

 appear by reduction. 



It is now known that the roots of several of our genuine aquatic 

 species bear root-hairs whose presence certainly testifies to the 

 absorbing activity of the roots and the hngering terrestrial habits 

 necessitating such organs. 



It must be noted that the foregoing discussion is purely theo- 

 retical and the intention has been to awaken interest in the study 

 of the aquatic flora rather than to offer a theory of origin for which 

 any originahty could be claimed. 



Experimental evidence may be obtained that roots are organs 



