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 A HISTORY OF HERTFORDSHIRE They are entirely aquatic, and have been termed by their chief British investigators, Messrs. H. and J. Groves, 'the pioneers of aquatic vegetation,' for they play the same part in the water as mosses do on the land. They may generally be found in stagnant water, lakes, and slow streams. The term 'stonewort' had been given to them on account of the amount of lime which some species secrete. Of the thirteen British species of Chara, three of To/ype/fa, and seven of Nitella, the following are present in Hertfordshire 1 Chara fragilis, Desv. vulgaris, L. ( = C. foetida, Braun) Tolypella glomerata, Chev. intricata, Leonh. Nitella translucens, Ag. flexilis, Ag. opaca, Ag. Fruits or seeds of Chara, probably of existing species, have been found in the ancient Hitchin lake-bed, but the genus existed in Britain long before the Pleistocene period, occurring in Jurassic times in beds of Wealden age. THE The Alga? are a very diverse class of plants, including such groups as the seaweeds, confervas, desmids, and diatoms. All are aquatic or hygrophilous, for some live on wet rocks or damp walls ; but very few of these are recorded for the county, nearly all our known species fre- quenting ponds and slow streams. The only rivers in which they have here been found are the Lea and the New River, all our other streams apparently being too rapid for them. Our knowledge of the Algas of Hertfordshire, exclusive of the Diatomaceae, is almost entirely derived from Hassall's British Freshwater Algce (1845), with a few records from the 'Transactions of the Hertford- shire Natural History Society. Hassall records 120 'species' as occurring in the county, nearly all being from the neighbourhood of Cheshunt, with a few from Hertford Heath and one from Royston, but many of his species are only varieties or forms of others, and a few cannot now be identified. Excluding these, the number recorded by Hassall is re- duced to eighty-six, and the total number now known is ninety-six, as in the following table CoCCOPHYCE.ffi Pleurococcus Porphrydium. Scenedesmus Pediastrum. Chlamidococcus Volvox. . . Hyalotheca. DESMIDIEJE Desmidium. Closterium. 6 I Micrasterias. 3 I Euastrum 3 I Cosmarium. 2 I Arthrodesmus. I 2 Staurastrium 2 I BOTRYDIACEJE 2 Zygnema 4 Spirogyra. . 1 1 I Sirogonium. 2 Mesocarpus. . 7 Staurospermum. 4 SIPHOPHYCE.S: Vaucheria. . 7 NEMATOPHYCE^ CEdogonium. 1 6 Bulbochcete. . I Hormiscia. . I Stigeoclonium. 4 Draparnaldia. 3 CYSTIPHOR.S: Chroococcus. i NEMATOGEN^ Oscillatoria. . 3 Lyngbya. . . i Rivularia. . i Gloiotrichia. . i RHODOPHYCE^: Batrachospermum 2 1 From the list, with- localities, of the Characeae of the South Midlands, given by Mr. James Saunders in Trans, Herts Nat. Hist. Soc. vol. ix. p. 25 (1896). 66