246 ^^ FRESil-WATER PLANTS. 



and a half-wa- ferently Contrived, the upper face exactly resembles common 

 ter grass. grass, but to support it on the water it has long cylinders, 



which are merely of a loosened skin of that kind, which per- 

 mits not water to penetrate. These are the air vessels, and 

 support the grass perfectly dry on the water, where it swims, 

 and defies both rain and wind, (see PI. VIlI,fiu;.l and 2, where 

 ^ are the air vessels ; fig. 4, PI. VII, being the water grass). 

 By this it may be seen how gradually the plants approach 

 to the state of perfect water plants, and if any other proof 

 is wanted, the potamogeton hicens, which has generally a 

 double stem, would show it. This grows constantly in the 

 water; it is small, and requires few wood vessels to bring 

 it support. It is almost wholly composed of vessels of air, 

 one wood vessel being between each row ; but it has a long 

 circular stem in the interior, bounded by the line of life, and 

 with a deep border of wood, up which the buds pass, as in 

 all the fern and potamogeton genera, though seme have several 

 Dissection of a instead of a single one. To this specimen I shall add a sort of 

 scirpus. scirpus growing always in water, having its air vessels next 



the rind, as in figs 3 and 6, PI. VI II, and merely threads 

 Dissection of tying the wood vessels in the middle, one to the other. The 

 air vessels of ^[j. yessels resemble in form those of tire water lily, and are 

 very different from the air cylinders in half-water plants, 

 which 1 should before have mentioned. These are divided 

 into compavtments, as in PI. VIII, fig. 5, ede, which re- 

 presents the veronica beccabunga, being half air vessels, 

 half wood, so that the air enters only alternately ; and they 

 have no hairs as in the water lily, the wood part being wholly 

 filled up. 

 Ns ptrsp^ra- The perspiration of aquatic plants was supposed to be 

 von m ulants. uncommonly copious. I have so often troubled the public 

 with the proofs of there being no perspiration in plants, that 

 J shall not here repeat them, but merely give an explanation 

 of the cause of the quantity of air found around almost all 

 fresh water plants. There is in the bark juice (that is, the 

 blood of the plant)- a glutinous matter, which when moved 

 catches the air in bubbles, 'vand will continually enclose'it, 

 and cover the whole leaf and plant with vesicles of air, 

 by which means it is prevented from sinking, let the rain 

 beat eyer so hard against it, or the wind attack it. I have 



seen 



