2. _The Ivel._--This district is S.W. of that of the Cam, and the Chalk Downs of that district are continued through it. Its rarer plants are _Melampyrum arvense_, which occurs only in one spot S. of Ashwell; _Smyrnium olusatrum_, which has been found near Baldock and Pirton; and _Silene conica_, which was found near Hitchin in 1875. The white helleborine (_Cephalanthera pallens_), the dwarf orchis (_Orchis ustulata_), and the musk orchis (_Herminium monorchis_) occur on the Chalk Downs. 3. _The Thame._--A very small tongue-like protrusion[a] of the extreme W. of the county, in which are the Tring Reservoirs. Two of the species confined to the district, _Typha angustifolia_ and _Potamogeton Friesii_, are water-plants which occur only in these reservoirs or in the canals which they supply. A rare poplar, _Populus canescens_, grows by the Wilstone reservoir, and the man-orchis (_Aceras anthropophora_) on terraces cut in the Chalk near Tring. 4. _The Colne._--A large district, comprising almost the whole of the western portion of the county. _Diplotaxis tenuefolia_, _Silene nutans_, and _Hieracium murorum_ grow only on old walls in St. Albans. Colney Heath is our only habitat for a very rare loosestrife, _Lythrum hyssopyfolium_, and also for _Teesdalia nudicaulis_, while there is but one other locality, a different one in each case, for four of its plants, _Radiola linoides_, _Centunculus minimus_, _Cuscuta epithymum_, and _Potamogeton acutifolius_. The pasque-flower (_Anemone pulsatilla_) grows abundantly on the Chalk slopes near Aldbury. The rarer orchids of the district are the bog-orchis (_Malaxis paludosa_), the narrow-leaved helleborine (_Cephalanthera ensifolia_), and the butterfly orchis (_Habenaria bifolia_). 5. _The Brent._--The smallest district, a protrusion[b] of the county in the S. entirely on the London Clay, and chiefly interesting owing to the presence of Totteridge Green and its ponds. In these ponds grow the great spearwort (_Ranunculus lingua_) and the sweet-flag (_Acorus calamus_), the former, however, not being indigenous. The star-fruit (_Damasonium stellatum_) formerly grew on Totteridge Green, and _Chenopodium glaucum_ at Totteridge, but neither has lately been seen. 6. _The Lea._--The largest district, comprising the whole of the E. of the county. The London rocket (_Sisymbrium irio_) occurs only in the old towns of Hertford and Ware; the true oxlip (_Primula elatior_) near the head of the River Stort; a very rare broom-rape, _Orobanche caerulea_, at Hoddesdon, where it is parasitic on the milfoil; and an almost equally rare bedstraw, _Galium anglicum_, on an old wall of Brocket Park. A rare trefoil, _Trifolium glomeratum_, is known only at Easneye near Ware; and Hatfield Park is our only locality for the water-soldier (_Stratiotes aloides_) except where it has evidently been planted. Two species, usually of rare occurrence, _Polygonum dumetorum_ and _Apera spica-venta_, are frequent in the district.