12 Plant-life of the Oxford District 



Owing to the damp state of broad alluvial tracts, a high day-temperature 

 may result in fogs in the lower levels, especially in autumn and early spring ; 

 while a sub-saturated atmosphere is often predominant for long periods in 

 these districts, though the atmosphere on higher better-drained ground may 

 be dry and clear. The scanty ancient population in British and Roman 

 times kept to the hills, probably for military reasons. The Saxons cleared and 

 cultivated the lower levels, and were largely restricted to the damp valleys 

 by the necessity for a permanent summer water-supply for their cattle. 

 The original urban area, as also adjacent villages (Marston, Cowley, Wolver- 

 cote, Binsey, Botley, S. Hinksey), follow the distribution of gravel-patches 

 with better drainage than that afforded by the clay areas. More modern 

 villa-residences again follow the hills where possible, as at Headington (now 

 the second most densely populated area in the county), Boar's Hill, and 

 Cumnor Hill, the further extension of which remains still limited by deficiency 

 of suitable water-supply, restricted to scanty underground streams or deep 

 borings, and with no reserves in case of fire. 



River Temperatures average from 46-5 F. as the minimum (March i), 

 to 55-2 as the maximum (Sept. i), with a conspicuous lag beyond the air 

 temperatures. In exceptional winters (1891,95) all streams freeze, including 

 the Thames. 1 In exceptionally dry summers all ponds and streams dry up, 

 except the rivers bringing water from beyond the district, 2 and a few minor 

 hill-springs. 



Soil Temperatures. The temperature is constant throughout the year 

 possibly at 66 ft., a depth too remote for the purposes of plant-life. Records 

 of the Radcliffe Observatory by Platinum Resistance Thermometers (1898- 

 1910) have been plotted for varying depths, showing that at the surface the 

 mean air-temperature is fairly followed, with maximum in July, though 1 

 colder in winter, and 3 warmer in summer. Lower levels show an increasing 

 lag at i^ ft., 3! ft., 6 ft., to a depth of 10 ft., at which the curve for river- 

 water is very closely approximated (or 46-5 min., 56 max.), and the local 

 occurrence of water in the gravel prevented deeper records being taken. 3 



Meteorological records have been also kept for the Botanic Garden 

 since 1861. (J. J. Manley, Magdalen College Lab.) These records show a wide 

 range of variation from the official data of the Radcliffe Observatory, only 

 half a mile away, probably owing to the fact that the garden is at a low level 

 on the alluvium near the river, and even more sheltered than the grounds of the 

 Observatory on terrace-gravel. 4 



Thus the rainfall minimum of 1921 was registered as 13-96 in. 

 The record cold of Feb. 4/5, 1917, is seen to have been due to local 

 blasts of blizzard intensity ; the minimum temperature on the grass that night 

 being 20, and a foot above the grass 4. 



The record hottest day, July 10, 1921, gave 85 in the shade, and 128-5 

 in the sun. In July 1921, the sun-temperature (absorption of radiant energy 

 measured as heat) did not exceed a maximum of 130-5, and the hottest day 

 ever registered at the gardens (Aug. 9, 1911) was 91-3 in the shade, and 

 132-5 in the sun, as opposed to 94-7 and 153-7 respectively, at the Radcliffe. 



1 A coach and four was driven on the ice from Folly Bridge to Iffley, and the river was free for 

 skating 10 miles below the City: Port Meadow flooded and frozen admitting skating to Wolvercote. 



2 In the drought of 1921 the Cherwell ceased to flow, the bed being dry at Islip in September, 

 and the water only held up locally by the lock at Iffley. 



s It is clear that this water was a running stream (Timms). 

 4 Guntlier (1916), Daubeny Laboratory Kegister, p. 172. 



