Further Observations on Heath A ssociation. i 21 
underground at average depths of about one and three inches 
respectively. 
VVe have already called attention to the removal of shelter by 
burning, resulting in a corresponding increase in evaporation from 
the soil, and this is probably enhanced by the hard surface-crust left 
after a fire. Moreover, the water-capacity of the soil is reduced 
owing to the destruction of the peat (cf. p. 122). Forms which root 
deeply will consequently he in a more advantageous condition as 
regards their water supply, until shelter is re-established, than 
shallow-rooting forms. This may in part account for the fact that 
U. nanus begins to sprout almost immediately, whilst in the case of 
Ciilluua, Erica, etc., there is considerable delay, often amounting to 
as much as a year. 
Observations made on an area burnt in June (FI) i.e., in the 
middle of the gl owing season, showed that about half the U. nanus 
sprouted in the same season, whilst the remainder were delayed 
until the subsequent April. On the other hand, in the case of a fire 
which took place later in the year, viz., in August (F 3), all the 
JJlex sprouted simultaneously in the following season. This may 
imply that if a fire destroys the U. nanus in the middle of its grow¬ 
ing period, the plants receive such a set-back that many are unable 
to rejuvenate immediately. There are some indications that other 
plants may be similarly influenced in their rejuvenation by the time 
of the year at which the fire occurs. We have, unfortunately, not 
been present at any of the fires on the Hindhead Common, so that 
we know nothing of the conditions of burning. Elgee (2, p. 8), 
however, has pointed out in this connection that “ where the fire 
passes over the heather with the wind behind it, the plants come 
again much earlier than when the burning has been against the wind 
and so more thorough.” 
(3) Destruction of the peat. 
It is to be noted that the peat on the oldest part of the heath 
(i.e. the north-western slopes of the central ridge) reaches a depth 
of from 3-5 inches, and here the Calluna and Vaccinium attain their 
maximum development. On the other hand, on all areas recently 
burnt, there is little or no peat, its place being taken by the charred 
crust already referred to. The destruction of the peat may well 
account for the poor growth of Calluna in the first years following 
a fire, and also for the predominance of Erica cinerea in the early 
phases, since the latter is favoured by a dry soil (7, pp. 104, 109). 
