260 OP AQUATIC PLANTS. 



uncertainty concerning several of the plants he examin- 

 ed. We ought to have been accurately informed w^hat 

 species of Poplar differed so remarkably in its power of 

 absorption from the Aspen, another of the same genus. 

 We ought likewise to have been told what Sun-flower, 

 what Nettle, Amaranth and Mallows were examined ; 

 for want of which information the authority of such ex- 

 periments is much impaired. 



From the foregoing observations we learn the impor- 

 tance of shading and watering plants newly removed, 

 cuttings, grafts, &c. and on the other hand the benefit 

 of heat and air to promote due perspiration and evapora- 

 tion. 



The perspiration of aquatic plants seems to be re- 

 markably copious. Of these some grow constantly im- 

 mersed in the water, as most species of Potamogeto7i^ 

 Pond- weed, Engl. Bot. t. 168, 297, 376, &c. Their 

 leaves are peculiarly vascular, and dry very quickly in 

 the air, withering in a very few minutes after exposure 

 to it. Their absorbing power seems equally great, so 

 that they appear to be continually, in their natural situa- 

 tion, imbibing and giving out a quantity of water much 

 greater than has been observed in land plants. Other 

 aquatics, as the Nympheece^ EngL Bot. t. 159, 160, 

 float with only the upper surface of their leaves exposed 

 to the air, which surface is so contrived that water will 

 scarcely remain upon it. These leaves, though extreme- 

 ly juicy, dry with great rapidity, as does every part of 

 the plants when gathered. It is probable that they im- 

 bibe copiously by their under sides, and perspire by the 

 upper. 



