and the cat's foot (_Antennaria dioica_) occur only on Royston and
Therfield Heaths; _Alisma ranunculoides_ and _Potamogeton coloratus_
only on Ashwell Common; and of the great burnet (_Poterium officinale_)
the sole record is that of a plant gathered near Ashwell in 1840.

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_)