Page:An introduction to physiological and systematical botany (1st edition).djvu/224

 194 the importance 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 evaporation.

The perspiration of aquatic plants seems to be remarkably copious. Of these some grow constantly immersed in the water, as most species of Potamogeton, 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 situation, imbibing and giving out a quantity of water much greater than has been observed in land plants. Other aquatics, as the Nymphææ, ''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 extremely juicy, dry with great rapidity, as does every part of the plants when gathered. It is probable that they imbibe copiously by their under sides, and perspire by the upper.