ORCHIDACE 2. 85 
into the sea, but be raked out at once upon the shores; and the Commissioners of 
Drainage should beware of letting fresh water into their districts, for the weed will 
inevitably enter with it and blockade the ditches.” Since Mr. Marshall wrote the 
above, in 1852, his prophecy has been fulfilled. Every ditch near the Cam or its source 
> 
is full of this weed, and the river itself is so choked up with it that it has become a 
serious nuisance, and we are not sure that the recent failures of the gallant University 
erew in the annual boat-race may not be attributed in some measure to the disad- 
vantages under which they train and practice on their own river. 
So strong is the conviction on the minds of those most interested in this question 
that some energetic means should now be taken to cleanse and purify the beautiful 
river of which generations have been so proud, that while we are now writing 
(August 1868) a committee of gentlemen has been formed, and is now actively at 
work collecting funds for that purpose, and organising improvements which will cost 
several thousand pounds to achieve. 
The Blodia is one of the best plants for the fresh-water aqua-vivarium. It grows 
rapidly, and oxygenates the water freely. In common with many other water plants 
—such as Chara and Vallisneria—it exhibits a very interesting structure under the 
microscope, and is an evident instance of the circulation of the sap in plants. The 
vessels, when properly placed under an object-glass of medium power, exlibit this 
curious phenomeuon very distinctly. 
ORDER LXXX._ORCHIDACE SA. 
Perennial herbs with fasciculate roots; in the terrestrial species 
often with two of the root-fibres enlarged into subglobular-ovoid or 
palmated tubers, a new one, terminated by a bud, being formed each 
year, and the one formed in the preceding year decaying; so that of 
the two tubers present one is exhausted, and the other contains 
nourishment to produce the stem of the succeeding year. Rootstock 
in the tuberous species blended with the tubers; in the others some- 
times creeping, or sometimes forming a cormlike enlargement sur- 
rounded by coats, so as to be intermediate between a corm and a 
tunicated bulb. Stem generally unbranched, leafy, or rarely with 
Scarious scales in place of leaves. Leaves often mostly radical, 
sheathing at the base, entire, parallel or cancellate-nerved, rarely 
reticulate; stem-leayes alternate or rarely opposite. Flowers perfect, 
irregular, in spikes or racemes or corymbs, rarely paniculate or solitary, 
each with a single bract. Perianth with an herbaceous tube adhering to 
the ovary; limb of six segments in two rows, usually all petaloid, 
free or more or less coherent, the anterior segment (which, from the 
ovary or pedicel twisting half round, is, in most of the british species, 
