92 MESSRS. J. А. WHELDON AND У. G. TRAVIS ON 
having overlooked them, rather than to what is in most cases really the fact, 
that they do not at present exist in the area. To botanists in other parts of 
the country, our lengthy list, which embraces a number of rare, and even 
new, species, might, at first sight, seem hardly consistent with the existence 
of the unfavourable conditions we have referred to. As a matter of fact, our 
list is more remarkable for what it does not contain than for what it does. 
(Juite a number of the species recorded have been found in a single locality 
only as the result of careful search ; and the greater proportion occurs in 
one or two specially favoured tracts situated as far from the great manu- 
facturing centres as the limits of the vice-county permit. It must also be 
remembered that many of our lichens are depauperate and ill-developed to 
such an extent that they are frequently difficult to determine ; nevertheless, 
poor as they are, they help to swell out the list and tend to give the reader 
the impression that the lichen-flora is better than it really is. 
А. Condition of the various Classes of Lichens. 
Corticole Species.—1t will be gathered from what has been already said, 
that it is the corticole lichens which have suffered most severely from the 
effects of atmospheric pollution ; and in fact the most striking feature of our 
lichen-flora is the comparative rarity of corticole lichens. А glance through 
our list will show that whole genera of bark-loving lichens, such as Calicium, 
Usnea, Ramalina, Graphis, Opegrapha, Arthonia, &c., are either totally 
absent, or very poorly represented. The conditions of atmospheric impurity 
in South Lancashire are such that corticole lichens have been brought to 
the verge of extinction in the vice-county. In other parts of the British 
Isles which are little affected by smoke, corticole species form a large 
percentage of the lichen-flora ; in South Lancashire, however, exclusively 
corticole or lignicole lichens only form about 15 per cent. of the species 
now existing. In considering this figure, allowance must be made for the 
fact that many of the species reckoned in our estimate have only occurred 
on prostrate trunks and dead twigs lying on the ground. 
Under normal conditions, the trunks of our older trees in өреп situations 
should be grey and shaggy with a growth of species of Usnea, Ramalina, 
Pertusaria, Parmelia, and Evernia, and any space left by the larger foli- 
aceous lichens should be more or less occupied by crustaceous lichens or 
bark-loving species of mosses and hepatics. As it is, we find that over the 
greater part of South Lancashire the tree-trunks are destitute not only of 
lichens but also of the common corticole mosses, which are affected by smoke 
in a similar manner to the lichens. On the shady side, perhaps, the bark is 
coated with a film of Green Algæ, and in some cases a greenish-yellow 
powdery crust is present, which represents a leprarioid state of undeveloped 
lichens. It is significant that where lichens cannot develop on vertical 
