242 <>N FRESH-WATER PLANTS. 



placed in regular vessels, surrounded bytheclearalbumen,and 

 containing the spiral vessels; andasthesecylinders are sprin- 

 kled all over the interior, and the interstices filled with pith : 

 they are known to be herbaceous, annual, or semiplants, 

 and their formation is understood, and they will still bear a 

 sort of comparison with trees and shrubs. But when the 

 whole arrangement is overturned, and plants are found pos- 

 sessing parts unseen before, as in the fresh-water plants; 

 or when in dissection the usual assortment of matter can no 

 longer be recognized, and neither wood, bark, spiral ves- 

 sels, &c. can any more be found, as is certainly the case 

 in the marine plants; then it is absolutely necessary, to be*^ 

 gin again the description, and by strict examination and trial- 

 discover anew the different uses, to which these parts can 

 be applied. 

 Monsieur du It is this cursory method of giving every diffei-ent sort of 

 woric'^'^''' plant together, which makes the work of Monsieur du 



Thouars so difficult to under: land: but it must be consi- 

 dered, that he delivered most of his book in lectures ta his 

 pupils, and probably exemplified them in the very best 

 manner by living specimens, which would explain them to 

 his hearers, though not to his readers. 

 Marine plants. As 1 intend to show in this letter the formation of fresh- 

 water plants, and in my next letter marine plants, it will 

 not be amiss to draw first a sort of comparison between 

 them, at once to prove the necessity of the arrangement juat 

 specified. Marine plants have no vessels. The whoje is 

 formed, whether thick or thin, by blebs, which allow of no 

 communication one wi^th the other : so that, if only a part 

 of a fucus is drawn out of the water, this part dies, the 

 rest not being capable of conveying to it any of its moisture^ 

 Each bleb has I believe a pore, but very difficult 'to find. 

 They have no peculiar air vessels, though much air is mixed 

 with the liquid within ; which, not being confined in vessels, 

 merely fills the blebs, and is retained by the cuticle of each 

 surface. The only vessel to be found in a fucus, ulva, &c., 

 is the line of life. They have no spiral wires, and of cours^ 

 no divisions of bark, wood, &c, ; and appear in short to 

 differ so much from every other plant, that their means of 

 a<.)uii5hment and existence must be wholly of another kind, 



and 



