ORDER NAIADE/E. GRASS- WRACK, POND-WEED. 515 



the creeks and salt-water lakes that pass inland. It is used, 

 where it is common, for packing glass bottles and earthenware ; 

 and also in cottages for making beds and cushions. The Pond- 

 weed (Potamogeton) of which a large number of species exist in 

 Britain, is another plant of this order ; its roots are fed on by 

 swans, which devour them with avidity, whilst its seeds are a 

 favourite article with ducks. One species is said to grow in the 

 Swiss lakes to the length of from ten to twenty fathoms, forming 

 as it were extensive subaquatic forests, in those vast natural 

 reservoirs; in Siberia the roots of the species which most 

 abounds there, are used as food by Man. The flowers of the 

 plants of this order have a calyx and corolla, each consisting of 

 two little scales, which very early fall off, and occasionally are 

 altogether absent. Some species are monoecious, whilst others 

 possess complete flowers. The stamens and carpels exist in a 

 small definite number; but this varies in the different species. 

 In Potamogeton there are in each flower four anthers, which, not 

 being elevated upon filaments, are said to be sessile ; and four 

 ununited carpels, which become four small nuts. On the other 

 hand, Zostera is monoecious. The plants of this order are most 

 abundant in countries beyond the tropics, although they are also 

 found near the equator. Potamogetons exist in almost every 

 ditch and swamp, as far north as Iceland. 



Order ORCHIDE^, or Orchis Tribe. 



692. The plants associated with the common Orchis in this 

 order, exhibit some of the most curiously-interesting modifica- 

 tions of structure, that any group in the vegetable world affords. 

 Most of them are remarkable for the resemblance between their 

 oddly-shaped flowers, and various objects with which they may 

 be compared; thus two species of this country are known as the 

 Bee-Orchis and the Fly-Orchis, from the similitude between 

 their flowers and those insects ; whilst others are known as the 



