ORCniDACEiE. 85 



into the sea, but bo ralcccl out at onco upon tlie sborcs ; and tlie Commissioners of 

 Drainage should beware of letting fresh water into their districts, for the weed ■will 

 ine\'itab!y 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 fall 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 

 crew 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 pui-ify the beautiful 

 river of wliich generations have been so proud, that while we are now writing 

 (August 1868) a comnaittee 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 Elodea 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 CJiara and Vallisnena — 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, exhibit this 

 curious phenomenon very distinctly. 



ORDER LXXX.— ORCHID ACE iE. 



Perennial herbs ■with fasciculate roots; in the terrestrial sjDecies 

 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 cormlOve enlargement sur- 

 roimded 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-leaves alternate or rarely opposite. Flowers perfect, 

 irregular, in spikes or racemes or corymbs, rarely paniculate or solitary, 

 each mth 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, 



