126 
F. F. Fritsch and F. j. Salisbury. 
In a later stage of development one or more of the branches of 
the cushions grow in a vertical direction, so that the flattened 
character is obscured. This is probably correlated with the estab¬ 
lishment of a root-system adequate to cope with the resulting 
increase of transpiration. That the prostrate habit, at first assumed, 
is due to exposure is shown by a comparison of the relative 
frequency, in situations of varying exposure, of plants (a) with all 
their branches prostrate, and ( b ) with one or more of their branches 
ascending (see Table VI). Such a comparison brings out the facts 
that (1) the percentage of prostrate plants is much greater in the 
more exposed situations, and (2) the number of specimens retaining 
the prostrate habit becomes smaller, the longer the interval that has 
elapsed since a fire, and consequently the greater the density of the 
vegetation. 
TABLE VI. 
Percentages of Completely Prostrate Specimens of 
Ulex NANUS on Burnt Areas. 
Direction of Prevailing Winds, S.W. 
Locality. Extreme exposure 
Less exposure 
(TOP OF slope). 
(base of slope). 
Top 
[F 1 (S.W. aspect) 71 
nil. 
fully 
< F 3 (W.N.W. aspect) 62 
52 
exposed. 
[F 5 (E.S.E aspect) 52 
15 
Sheltered by ( 
F 2 (S.W. aspect) 11 
5 
opposite slope. ( 
F 3 (E.S.E. aspect) 44 
30 
In view of 
the fact that Calluna develops i 
n the main from 
seedlings which 
arise a year or more after the 
fire (cf. p. 135), a 
certain amount of shelter has already been created by the Ulex when 
the Calluna first appears. Hence it is not surprising that the latter 
exhibits a prostrate habit much less commonly, though here again 
the effect of increased shelter is clearly exhibited (cf. Table VII). 
TABLE VII. 
Percentages of Completely Prostrate CALLUNA on 
Burnt Areas. 
Locality. Extreme exposure Less exposure 
(top of slope). (base of slope). 
53 ... 7 
40 ... 39 
26 ... nil 
F 1 
F 3 
F 5 
