PAULSON : NOTES ON THE ECOLOGY OF LICHENS. 285
covering several square feet of soil; the only limit to the
area covered being the size of the original bare patch of
ground. Fusion readily takes place on contact as the above
lichens have no definite upper or lower cortical tissue. The
rate of growth of any individual patch is not known to us, the
facts noted being, that patches of ground which were quite
bare in the early summer of 1917 were completely covered
with lichen growth on the 10th of November of the same year.
Xanthoria parietina, Parmelia physodes, and Parmelia fuliginosa
can be observed starting from a definite centre of growth and
the increase is measured through this centre to a constantly-
enlarging circumference.
Physcia ciliaris D.C., on living branch of hawthorn (fig. 3),
is an example of a lichen producing numerous apothecia within
a period of four years, as shown by the number of the annual
rings of wood. It is quite possible, by counting these rings,
to determine approximately the age of the lichen. I have not
found lichen growth on branches, even of elder, of less than one
year's growth.
The lichens (fig. 4) Parmelia fuliginosa (3.5 cm. across the
widest diameter) and P. saxatilis (3.3 cm. across) are upon a
portion of the bark of a plane tree picked up at the time the
bark was being shed. They represent a growth of less than
two years.
Fig. 5 is a photograph of lichens on another branch of haw-
thorn. They seldom bear apothecia in south-eastern England.
The twig has three annual rings of wood. It was apparently
not living when collected, but the wood exhibits no marked
signs of decay. I estimate that the lichen-growth is that of
a period of less than six years. In the case of the middle lichen,
Parmelia physodes, it is becoming overgrown with P. fuliginosa,
even at that early stage.
A change in conditions of environment will arrest lichen
growth and kill plants in a very short period of time. This
has been made very evident in many woods where extensive
felling of trees for war purposes has suddenly increased the light-
intensity. Lecanora varia, which often covers tree trunks in
moderate shade, may now be seen, after a few months' exposure
to increased light, to have lost its green colour and become grey
or nearly white—so much so that the trees appear from a short
distance as though they had been splashed with whitewash.
Sun-loving lichens show, in the same way, a distinct change in