226 
ST. ALBANS AND ITS NEIGHBOURHOOD. 
the larger ponds (Smitli, “ Notes on the Vegetation of Ponds,” 
‘ Naturalist,’ Oct. 1903). At the margin of the marshy ground 
and above the water-level stands the zone of rushes which forms 
a sharp boundary between the marsh-association below and the 
furze-heath above. This is followed by a broad zone of swampy 
turf which, though rarely, if ever, covered by the water, is 
completely saturated. The chief of the plants comprising this 
zone are the marsh-stitchwort ( Stellaria uliginosa), milkmaid 
( Cardamine pratensis), marsh - marigold ( Galtha palustris ), 
strawberry clover ( Trifolium fragiferum), Triglochin palustre, 
Eleocharis palustris, and Alopecurus geniculatus ; more rarely 
the sneezewort ( Achillea ptarmica) and waterblinks ( Montia 
fontana). Next follows a zone of varying width, but usually 
narrow and constituted by plants rooted in very shallow water; 
here in varying frequency are to be found the water-dropwort 
(( Enanthe fistulosa), water-horsetail ( Equisetum limosum), bur- 
reed ( Sparganium ramosum), water-celery (Sium nodiflorum), 
yellow flag ( Iris pseudacorus) , flowering rush ( Butomus 
umbellatus), and bog-bean ( Menyanthes trifoliata). Owing to 
the shallowness of the stream at some points this zone may 
extend to the further bank, but here and there the water 
becomes sufficiently deep to mark a fourth zone comprised by 
such species as are more or less entirely submerged; of these 
the commonest are pondweed ( Potamogeton natans), water- 
starwort ( Callitriche verna), Canadian water-weed (Elodea cana- 
dense ), Glyceria fluitans, and Ohara hispida. The last named 
forms the home of numerous snails and leeches. 
To the action of this stream, when vastly more considerable, 
was no doubt due the formation of the sand and gravel which 
characterize the major portion of the heath. With the presence 
of these are associated such plants as allseed ( Uadiola mille- 
grana), dwarf furze (Ulex nanus) , wood cudweed ( Gnaphalium 
sylvaticum), chamomile (Anthemis nobilis ), and the bird’s-foot 
( Ornithopus purpusillus ). 
The south end of the heath differs greatly from the portion 
traversed by the stream, for here the soil contains a large 
amount of humus, and the occurrence of a new association of 
plants is at once apparent. Small patches of needle-furze 
( Genista anglica ) alternate with clumps of dwarf furze ; scattered 
amongst these are the upright tufts of the moor-grass and bog- 
rush ( Nardus stricta and Juncus squarrosus), whilst red-rattle 
(.Pedicularis sylvatica) and dog - violets ( V. canina ) furnish 
bright patches of colour. The two ponds in this part of the 
heath are also of interest, not only for the zonation they exhibit, 
but also for the richness of their algal flora. 
No-Man’s Land and Harpenden Common are linked together 
by the fact that both are traversed by the same dry river-bed 
with extensive deposits of valley-gravel. Both show signs of 
having become drier in recent years, a feature the more marked 
at Harpenden, where such plants as heather, red-rattle, rushes, 
