ST. ALBANS AND ITS NEIGHBOURHOOD. 
227 
and ferns have either disappeared or are rapidly doing so. On 
the more peaty common of No-Man’s Land the dwarf furze, 
needle-furze, and red-rattle occur in some quantity, but are 
absent at Harpenden, the last-named species having died out 
there some 15 years ago. 
Of the less common plants met with here Potentilla argentea, 
cat-mint ( Nepeta cataria), and Filago minima do not occur at 
Harpenden, whilst common to both are Moenchia erecta, bird’s- 
foot ( Ornithovus purpusillus), Trifolium filiforme, chamomile, 
and other plants characteristic of gravelly soils. 
Among the grasses it is of interest to note the rapid spread 
during recent years on both commons of Koeleria cristata and 
Triodia decumbens. 
A striking object-lesson for the oecologist is furnished both at 
Harpenden and No-Man’s Land, where in each common there is 
a small area from which clay was at one time dug, thus 
exposing the chalk below. These “ dells,” as they are popularly 
termed, have now been left for many years undisturbed. We 
thus find in each of these two commons an isolated patch of 
calcareous soil surrounded oil all sides by clay and gravel. In 
both cases a characteristic chalk association has sprung up, the 
most conspicuous members of which are salad burnet ( Poterium 
sanguisorbd) , purge-flax ( Linum catharticum), autumn gentian 
(Gentiana amarella), flea-bane (Frigeron acre), and the carline 
thistle (Carlina vulgaris). The total absence of these species 
from the surrounding common renders the contrast striking. 
The chief feature of interest at Harpenden, and one possibly 
associated with the lowering of the water-level, is the struggle 
still in progress between the furze and the bracken. Little more 
than ten years ago the greater part was a furze heath with a small 
amount of bracken, chiefly at the higher southern end; this latter 
has gradually spread downwards and northwards, till now, when 
seen in summer, the major part exhibits large areas where isolated 
and dishevelled furze-bushes, like almost submerged islands in 
a sea of green, alone remain to tell of what was once a dense 
stretch of gorse, relieved in the more open parts by the stately 
tussocks of Aira csespitosa. Bernard’s Heath, where the soil is 
a gravelly loam, is mainly covered by brambles, gorse, and wood- 
sage ; and, perhaps in part owing to its nearness to the city, has 
a comparatively poor flora. 
The alien phanerogams of the district are chiefly of interest 
by reason of the rapid spread of several species. Coleman in 
his manuscript Flora (1839) refers to Veronica buxbaumii as 
rare ; at the present day it forms one of the commonest of our 
weeds on arable land. The recently introduced Matricaria suaveo- 
lens affords an example of a species which is spreading at the 
present time. This portion of Hertfordshire in no way escaped 
the sudden invasion of its streams b jFlodea canadensis subsequent 
to its introduction into England about 1841 ; and the monkey- 
flower (Mimulus luteus) bids fair to become equally ubiquitous. 
