422 TRANSACTIONS OF THE [Sbss. lxxix. 



Hydrilla verticillata, Caspar y, in Great Britain. 

 By Arthur Bennett, A.L.S. 



(Read 14th January 1915.) 



In July Mr. W. H. Pearsall sent me a box of aquatics 

 from Estwaite Water, Lancashire. Among them I was 

 o-lad to find Naias flexilis, Rostk. et Schmidt (an addition 

 to the English flora), along with Potamogeton obtusi- 

 folius, Mert. et Koch, pusillus, L. and crisjpus, L., Calli- 

 triche autumnalis, L., and a plant which seemed to 

 answer to the above, but I wished the name confirmed, 

 and sent it to Kew ; Mr. N. E. Brown replied, " Hydrilla 

 verticillata, an interesting find " ; and to the British 

 Museum, whence Mr. Wilmott wrote, " Hydrilla verticillata 

 certainly." 



It is a close ally of Elodea canadensis, and many 

 botanists consider that the genera Anacharis, Elodea, and 

 Hydrilla should be combined and form one genus. 



It differs from Elodea in having " only one leaf to the 

 spathe," but this character is not easily observed. Our 

 plant seems to give few winter-buds which throw out roots, 

 the stem being slender, with opposite leaves and short 

 internodes, the leaves dark green in colour; these are 

 succeeded by longer internodes, with the leaves linear- 

 acuminate in threes and fives (or occasionally in fours), 

 the uppermost leaves being whorled, pellucid : the whole 

 plant is paler and more pellucid than Elodea. If the 

 figure in English Botany, ed. 3, t. 1446 (1869) of the leaf 

 margin is correct, then I think Hydrilla differs by the 

 marginal transparent cells, which are extra-marginal and 

 can hardly be called serrations. 



The leaves are acute, narrower than Elodea, and the 

 habit of the plant is much more gracile, and does not grow 

 in the masses (at least in England) that Elodea does. 



Mr. Pearsall observes : " The plant is very scarce ; it never 

 grows with Elodea, but almost invariably I get a plant or 

 two (at most) among a mass of Naias and Gallitriche. 

 The depth of water it grows in is from 6 to 8 feet. It is 

 recognisable at a glance : the leaves are usually in fives or 

 threes, lighter in colour, and more acute at the apex ; the 



