VI] FIXITY OF POSITION IN PLANTS 87 



But some few plants of larger growth, such as the 

 Avell-known Gulf- Weed {Sargassum), may also float 

 unattached, growing and taking nourishment at the 

 surface of the open sea. This is, however, excep- 

 tional, and for the most part the more advanced 

 vegetation of salt and fresh water is attached to the 

 substratum, though the reason for this is not directly 

 one of nutrition, for they still feed by absorption at 

 the general exposed surface. The fact that in the 

 sea, in large bodies of fresh water, and in streams the 

 medium in which the plant lives is always changing 

 owing to the tides, currents, and general water-move- 

 ments, materially aids nutrition, for by this means 

 fresh supplies of inorganic materials are continually 

 being presented to the floating plants. 



Coming finally to the vegetation of the land, we 

 find it composed essentially of plants rooted in the 

 ground. The extent and effectiveness of the attach- 

 ment are well seen when the roots are washed clear 

 of the soil, as on the shore about high-tide mark, or 

 along the banks of a stream that has been in spate. 

 The reasons for this fixity by the root are primarily 

 mechanical and nutritive. It is by such anchoring of 

 the plant firndy in the soil that it is possible for it to 

 raise its limbs and leaves upwards to expose them to 

 light and air. And, as every one knows, this is a 

 condition necessary for the successful nutrition of the 

 green plant. But, secondly, it is in the root-system 



