1097. The habits of aquatics are various, in regard to the depth of water: thus, some aquatics float always on the surface of the water, as Lemna ; while, others are either partially or wholly immersed. Such aquatics as grow in the depths of the sea are not influenced by climate; but such as are near the surface are influenced by climate, and have their habitations affected by it. The moisture, or mode of watering, natural to vegetables, is a circumstance which has a powerful influence on the facility with which plants grow in any given soil. The quantity of water absolutely necessary for the nourishment of plants, varies according to their tissue; some are immersed, others float on its surface ; some grow on the margin of waters, with their roots always moistened or soaked in it; others, again, live in soil slightly humid or almost dry. Vegetables which resist extreme drought most easily are, 1. Trees and herbs with deep roots, because they penetrate to, and derive sufficient moisture from, some distance below the surface; 2. Plants which, being furnished with few pores on the epidermis, evaporate but little moisture from their surface, as the succulent tribe. The qualities of water, or the nature of the substances dissolved in it, must necessarily influence powerfully the possibility of certain plants growing in certain places. But the difference in this respect is much less than would be imagined, because the food of one species of plant differs very little from that of another. The most remarkable case is that of salt-marshes, in which a great many vegetables will not live, whilst a number of others thrive there better than anywhere else. Plants which grow in marine marshes, and those which grow in similar grounds situated in the interior of a country, are the same. Other substances naturally dissolved in water appear to have much less influence on vegetation, though the causes of the habitations of some plants, such as those which grow best on walls, as Peltaria, and in lime-rubbish, as Thlaspi, and other Cruciferï¾µ, may doubtless be traced to some salt (nitrate of lime, &c.) or other substance peculiar to such situations.
