INTRODUCTION Xxl 
bark and thus impede the aeration of the tree. On the other 
hand, it is suggested that they probably protect the tree from 
extreme cold. They do, however, indirect harm by providing 
harbourage for insect pests. 
PHYLOGENY AND CLASSIFICATION. 
Lichens as regards both their symbionts are polyphyletic in 
origin, the alge which form gonidia belonging to widely separated 
groups, and the fungi which form lichen-hyphe being derived 
from the two great subclasses Basidiomycetes and Ascomycetes. 
All the British lichens are derived from the Ascomycetes, 
- and they are again polyphyletic within the subclass. They fall 
into two great series :—— 
I. Gymnocarpee, in which the fruits have more or less 
open discs. 
II. Pyrenocarpez, with closed fruits. 
The Gymnocarpee are divided into three subseries :— 
1. Coniocarpine, characterized by a “ mazedium” type 
of fruit, and, in most of the families, by a primitive 
thallus. The derivation is doubtful. 
2. Cyclocarpinez, with open fruits corresponding to those 
of the Discomycetes. Their fungal ancestors are to 
be sought for among the Pezizine and most probably 
in the family Patellariacez. 
3. Graphidinee. The fruits are characterized by a 
narrow generally elongate disc. The nearest allies 
among fungi are the Hysteriacez. 
In the arrangement of the. various Orders or families, special 
attention has been given to the development of the lichen-plant 
as a whole within each series. 
The Coniocarpinee are a subseries apart, intimately con- 
nected with fungi and have been placed first. The almost 
closed form of the fruit suggests, however, aflinity with the 
Pyrenocarper. 
In the Cyclocarpinex, the first group includes those families 
in which the fruit is highly developed, the apothecia with few 
exceptions having a thalline margin (lecanorine). It was 
