136 F. E. Fritsch and E. J. Salisbury. 
where the crust was left intact, the radicles of the majority failed 
to penetrate; moreover, it was noticed that the few successful ones 
were those which had lodged in small crevices. We may add that 
the radicles of many of the unsuccessful seedlings exhibited an 
upward curvature, away from the burnt crust. Possibly the earlier 
appearance of U. nanus seedlings, as compared with those of the 
other heath representatives, is due to a greater toleration of the 
inhospitable surface. It is probable that the gradual increase in 
the coating of gelatinous algae may after some time facilitate the 
penetration of the radicles of germinating seedlings. 
The rate of growth of the shrubby members depends largely 
upon conditions of shelter, and consequently corresponds more or 
less to the phase of development of the heath. Thus, as already 
mentioned, the upright shoots of U. nanus often exhibit dying bach 
on burnt areas, whilst the prostrate shoots grow at an average rate 
of slightly over two inches per annum. Later, when more shelter 
has been established (CUE phase), the average rate of growth of 
the upright shoots (based on a large number of specimens) is 2-5 in.; 
in the final (CU) phase, the growth of such shoots amounts on an 
average to 1’5 in. The Callunas and Ericas develop later than the 
Ulex nanus on the burnt areas, and related to the shelter already 
established, their growth is at first very rapid. In the CUE phase 
the average rate of growth for Calluna is 1’235 in., and in the CU 
phase 1 • 154 in. It will be seen that the falling off in the rate of 
growth of older specimens of Calluna is much less marked than in 
Ulex nanus which is probably correlated with the fact that tall 
specimens of the latter develop as isolated plants in the CU, whereas 
the tall Calluna grows en masse. 
As the shrubby members become established, most of the 
herbaceous forms are ousted, Air a flexuosa, Molinia ccerulea and 
Carex pilulifera being almost the only ones that remain in any 
quantity; Polygala vulgaris and Potentilla erecta are occasionally 
present, whilst Blechnum spicant is a rare form occupying the damper 
hollows. The ultimate phase with dominant Calluna and Ulex 
nanus often includes drawn-up Erica cinerea and E. tetralix, with 
etiolated specimens of Pteris and Vacciuium (cf. 5, p. 178). In the 
wetter parts with abundant peat, where the Vacciuium grows as 
rapidly as the Calluna, the former becomes a co-dominant and 
exhibits no etiolation. Actinometric data show that the light- 
intensity at the soil-level in the tall Calluna- zone is only about one- 
fifth of that outside. 
