6. Rainfall.—Twelve years is much too short a period to give a trustworthy mean for such a variable element of climate as rainfall, and five stations are much too few to deduce an average from for Hertfordshire. The average rainfall at a varying number of stations for the sixty years 1840 to 1899 (from one station in the first decade of this period to twenty stations in the last decade) was 26.15 inches. In the driest year (1854) 17.67 inches fell, and in the wettest (1852) 37.57 inches. Spring has 5.40 inches, summer 6.97, autumn 7.87, and winter 5.91. The driest months are February and March, each with a mean of 1.65 inch; April is but very little wetter, having 1.69. The wettest month is October, with 2.96 inches, and the next is November with 2.56. The mean number of days of rain in the year, that is of days on which at least 0.01 inch fell, for the thirty years 1870-99, was 167. Autumn and winter have each about six more wet days than spring and summer. The rainfall is greatly affected by the form of the ground, the southern and 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, 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] 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] 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.