The indigenous flowering plants of Hertfordshire number 893 species, 679 being Dicotyledons and 214 Monocotyledons. If to these be added 199 aliens, etc., the total number of species recorded is brought up to 1,092. The flora is essentially of a southern type, the northern species being few in number. Owing to the dry soil, xerophiles largely prevail over hygrophiles. _The Ferns_ and their allies the horsetails and clubmosses are not well represented, both the soil and the air of the county being too dry for them. Another cause for the present scarcity of ferns is the proximity of Hertfordshire to London, for they have been uprooted and taken there for sale in cart-loads. We have twenty-four species of ferns and fern-allies, but not one really rare. The principal varieties are _Scolopendrium vulgare_, var. _multifidum_; _Athyrium filixfaemina_[c], var. _convexum_; and _Polypodium vulgare_, var. _serratum_. _Equisetum silvaticum_ is our rarest horsetail; and our only clubmoss is _Lycopodium clavatum_. _The Mosses_ are much better represented than the ferns, 175 species having been recorded. The bog-mosses are represented by six species--_Sphagnum intermedium_, _cuspidatum_, _subsecundum, acutifolium_, _squarrosum_, and _cymbifolium_. _Tetraphis pellucida_ occurs in Sherrard's Park Wood, and _Polytrichum urnigerum_ in Hitch Wood. _Seligeria pusilla_ has been found in an old chalk-pit in Brocket Park, and _S. paucifolia_ on chalk nodules in the Tunnel Woods near Watford. _Campylopus pyriforme_ occurs in Berry Grove Wood, Aldenham, and _C. flexuosus_ in Dawley's Wood, Tewin. Of _the Liverworts_ (_Hepaticae_) forty-four species are known to occur; and the Stoneworts (_Characeae_) are represented by seven species--two of _Chara_, two of _Tolypella_, and three of _Nitella_. _The Algae_ have been pretty fully investigated, especially the _Diatomaceae_, of the 252 species of Algae known to occur in the county, 156 belonging to that interesting family of microscopic plants. As an illustration of their minute size it may be mentioned that a single drop of water from the saucer of a flower-pot at Hertford, mounted as a microscopic slide, was found to contain 200,000 separate frustules of _Achnanthes subsessilis_, and it was estimated that these occupied only one twenty-fifth part of the drop. Both species of _Chlamidococcus_ (the old genus _Protococcus_), _C. pluvialis_ and _C. nivalis_ occur; and the pretty _Volvox globator_ has frequently been found.