ortion in the N. is in that of the Ouse.

In Pryor's _Flora of Hertfordshire_, published by the Hertfordshire
Natural History Society in 1887, which should be referred to for full
information on the botany of the county, these botanical provinces are
again divided into districts, the Ouse into (1) Cam, (2) Ivel; and the
Thames into (3) Thame, (4) Colne, (5) Brent, (6) Lea; both the larger
provinces and the smaller districts thus being founded on the natural
divisions of a country, drainage areas or catchment basins.

In the following brief notes a few of the rarer or more interesting
flowering plants of each district are enumerated.

1. _The Cam._--This is the most northern district. It is almost entirely
on the Chalk and is very bare of trees. The few plants which are
restricted to it are very rare. A meadow-rue, _Thalictrum Jacquinianum_,
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 dif