Further Observations on Heath Association. 131 
“ poor Calluna .” On certain neighbouring heaths ( e.g ., at 
Frensham, 3, p. 155) this condition, with a poor growth of Calluna 
and an abundant lichen-flora, in which a species of Cladonia 
(probably C. sylvatica) predominates, appears to be a relatively 
permanent state. In the tall Calluna on Hindhead C. sylvatica is 
the only common lichen, apart from a fairly abundant growth of 
Parmelia physodes Ach. upon the older stems and branches and a 
small amount of Cladonia pyxidata (cf. 1, p. 68). 
Mosses are to be found in the very earliest stages and these 
include Ceratodon purpureum Brid. which is very abundant, together 
with small patches of Tortula subulata Hedw. and Funaria hygrome- 
trica Sibth. In slightly later stages these are associated with 
Campylopus brevipilus B. & S. and Polytrichum piliferum Schreb. 
The mosses, although locally forming continuous sheets, are not 
uniformly distributed over burnt areas as is the case with the algae 
and lichens. About two or three years after a fire Ceratodon tends 
to disappear, and for a time mosses are quite insignificant as 
compared with the abundant lichens. It is interesting in this 
connection to note that Elgee writing of the “ fat moors,” says 
that some swiddens have been almost covered with lichens, chiefly 
of the genus Cladonia (1, p. 45). 
In the final stages of the heath however, mosses are abundant; 
the most conspicuous forms are Hypnum cupressiforme L. var. 
ericetorum B. & S., Lencobryum glaucum Schimp., and Dicranum 
scoparium Hedw., the first-named in particular attaining a great 
development where light penetrates through the Calluna- canopy. 
On the land-slides on the south-east slopes of the central 
ridge, where the soil consists of numerous pieces of broken rock 
mingled in a sandy matrix, the terrestrial form of Zygnema 
( Zygogonium) ericetorum Hansg. forms extensive dark purple sheets 
binding together the surface-layers of the sand, but, possibly owing 
to the mobile nature of the substratum, it does not as yet appear to 
have led to any permanent fixation of the surface. There is, 
however, reason to believe that this alga is of relatively recent 
occurrence on the land-slides. This form is peculiarly suited to 
such arid situations, for not only are its cells remarkably thick- 
walled, but, in conditions of drought, it exhibits various modifications 
tending to prevent excessive loss of water and at the same time 
permitting of rapid recovery when wet weather sets in. 
At one or two points extensive patches of the Z. ericetorum are 
becoming overgrown by bright green filaments of Hormidium 
