THE BOG-MOSSES OF ESSEX. 113
In addition to these lists there are specimens of bog-mosses
collected in Essex, in the following Herbaria:—
(9) British Museum of Natural History.
(io) Essex Field Club Herbarium.
(Including the collections of Messrs. English and Varenne.)
All of these sources of information have been consulted in
drawing up the list, and the records duly incorporated.
The Sphagnaceae so far as their vegetative structure is
concerned, form a highly specialized group, and are particularly
well adapted for life in situations where there is an abundant
and constant supply of water contained in shallow basins and
flowing but slowly, it at all. The majority of the species appear
to thrive best in the presence of abundant light. These some-
what peculiar conditions are realised in few places in our county,
and only, it would seem, where a porous stratum forming the
top of a hill meets a non-porous stratum as one goes down the
slope. Except where drainage, either for agricultural purposes
or for building, has altered the conditions, they are fulfilled
wherever the Bagshot Sands overlie the Boulder Clay or the
London Clay, and in some places where Glacial Gravel forms
the crown of the hill, the lower portion of which is of Boulder
Clay or London Clay.
I have collected Sphagna in the following localities where
London Clay underlies Bagshot Sands—Warley, Norton Heath,
High Beach, Galleywood Common, Mill Green, and Hainhault
Forest. In all the other localities where I have found these
plants growing, the drift maps show Glacial Gravel (Post-glacial
at the Rising Sun, Walthamstow), except at the locality in
Thomas' Quarters, Epping Forest, and there the map must be
at fault. I have not found any Sphagna, definitely associated
with the Boulder Clay that forms so large a part of the surface
of the County—possibly owing to the presence of a fairly large
proportion of lime in that formation.
[Note.—In the following list the names in brackets refer to the collector of
the specimen quoted, I have examined those from localities followed
by the sign ! and have collected a specimen in the localities followed
by !!]