84 ENGLISH BOTANY. 
a foreign country, have been carefully weighed by Mr. Marshall, and he does no 
hesitate to say that he believes it to be a genuine foreigner, brought here most pre 
bably from North America, He thinks that since its first discovery in the lock 
Dunse Castle in 1842 it has gradually been drifted by streams or canals into most of | 
our Midland rivers. Its extraordinary increase of late years is an argument in favou 
of its foreign origin; for, if it be not a new plant in our own rivers, how is it that 
never before exhibited this remarkable property of rapid increase? If it be a native 
this new faculty has been recently imparted to it—which seems absurd. But allowing 
as we must, that it is an introduced plant, and did not exist at all in our rivers till, 
few years ago, we are curious to know how it got here. Possibly some arder 
botanist, amongst specimens received from other lands, may unwarily have let f 
into a piece of water some fragment of this inveterately growing weed ; or we ma 
receive Mr. Marshall’s suggestion, that it was brought over in some American timbe 
used in the construction of some of the numerous railways which culminate at ¢ 
near Rugby. We know that American timber is floated in rafts down the rivers, i 
which case fragments of the weed would adhere to it, or seeds might find their w 
into the crevices of the wood, and if but one fragment of the plant or but one seed im” 
some moist cranny retained its vitality till it reached England, it is quite sufficient . 
account for the myriads of individuals now existing in our rivers and ponds. “ Indeed, 
from the circumstance of all the plants hitherto found being of one sex, the theory 
its propagation from a single seed or fragment is rendered more probable than by 
supposing a number of seeds or fragments to have been imported.”’ But an interestin 
question oceurs as to how the troublesome weed came to establish itself in the 
the classic stream which ought to be safe from such intruders. Mr. Babington tel 
us that in 1847 a specimen from the Foxton Locks was planted in a tub, in the 
Cambridge Botanical Garden, and in 1848 the late Mr. Murray, the curator, placed 
a piece of it in the Conduit stream that passes by the new garden. In the follow 
of the plant, he was informed that it had spread all over the ditch. “ From this 
point,” says Mr. Marshall, “it doubtless escaped by the waste-pipe across the 
~~ Eee 
a em 
navigation and drainage.” When Professor Gray, of Boston, U.S., was at Cambridg 
this circumstance was mentioned to him, and he expressed his surprise, as t 
waters, and especially the excess of lime present, may be favourable to its devele 
ment and form an inexhaustible supply of inorganic food for its nourishment 4 
support. The great practical question arising from all this seems to be, How is t 
London kitchens, or (more remarkable still) like the exotic molluse (the Dreisse 
polymorpha) which has now spread itself through the canals of this country, we® 
conclude it has fairly established itself amongst us, never to be eradicated. All 
shall be able todo is to try and keep it down; and in order to effect this, it 
should not be left in the rivers after it has been cut, in the hope of its finding its ' 
