• N FRESH-WATER PLANTS. 245 



require it to turn themselves, and expose their upper sur- 

 faces to light and heat ; which when they lie on the water 

 (as in absolute water plants) is not necessary. The leaves 

 also of the plants just mentioned, the veronica and menyan- , 

 thes, have in their peduncles many air, and but one set of 

 wood vessels ; but the most curious part is a sort of perfora- 

 tion in the bottom of the peduncle, which is formed between 

 the air vessels and lower cuticle to contain air, and support 

 the leaf stem upright above the water. 



May we not therefore conclude, that the air vessels are Air vessels 



only intended to support the stem upright in a different "^'"S,^"^^* 



•' _ ' ^ . commodate to 



element, and to raise it above, or depress it below the water, their element, 



as the situation of the plant required? Could I but 

 procure a plant, that had by change of climate become a 

 sort of water plant (which I should sup])oseis possible) it 

 would then be easy to see, whether the air vessels would 

 form themselves to accommodate their structure to the 

 element in which they reside. I cannot but be persuaded 

 I have seen changes as great : will not a tree by degrees 

 change its time of shooting? will not a plant often seek a 

 more agreeable soil if near it ? But time may enable me to 

 show this in a more conspicuous manner. I am so fearful 

 of advancing a single step beyond what specimens will 

 absolutely disclose, that I would far rather leave a fact un- 

 accounted for, than pass beyond what dissection will really 

 justify or bring proofs of. 



As it will not be possible, on account of the numerous DifFerence be 

 figures, to give all I have to say on fresh-water plants in ^w^^i^i'eal 

 this letter, I shall conclude by showing the difference be- 

 tween a real water grass and a half-water one, or that which 

 at one time of the year lies on water. The former is entirely 

 composed of rows of air vessels, which between every three 

 or four have a row of bark studded with wood vessels, very 

 small, and as usual half wood, half albumen, but no spiral 

 vessels. When the flower shoots, it is in thick threads from 

 the root ; slipping up between the leaf and its outward cuti- 

 cle, as in all grasses. This thread ib the line of life, with 

 some wood surrounding it, and is a fresh proof, if it was 

 required, that the female proceeds from this line, and the 

 male from the wood, But the half-water grass is very dif- 

 ferently 



