western hills attracting the rain, which chiefly comes from the S.W., so
greatly that with a mean annual fall of about 26 inches there is a
difference of 3½ inches between that of the river-basin of the Colne on
the W. and that of the river-basin of the Lea on the E., the former
having 28 inches and the latter 24½. The small portion of the
river-basin of the Great Ouse which is within our area has rather less
rain than the average for the county.


IV. FLORA AND FAUNA

In his _Cybele Britannica_, H. C. Watson divided Britain into eighteen
botanical provinces of which the Thames and the Ouse occupy the whole of
the S.E. of England. The greater part of Hertfordshire is in the Thames
province and a small portion 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_,