THE THALLOPHYTE DIVISION 509 
much economic importance. They may be defined as algo- 
fungal air-plants. Although made up of plants which belong 
to different classes of Algze and Fungi, which therefore on 
theoretical grounds might require to be assigned each to its 
own class, lichens are in practice more conveniently treated 
as compound organisms forming an artificial group by them- 
selves. 
Fic. 337.—Mushroom-lichen (Cora pavona, Mushroom-lichen Family, 
Coracee). A, top view of fruit-body, natural size. B, under side 
showing hymenium (hym). (Strasburger.)—On trees in the tropics. 
Their dual nature, indeed, doubles the difficulty already encoun- 
tered in trying to associate different types of fungi with their nearest 
algal kin. It is of course always desirable to express as well as we 
can our knowledge of resemblances and our views of kinship; but 
all this may be done effectively by using names like Ascolichenes 
and Basidiolichenes (suggestive of relationship between the lichen- 
fungi and their non-symbiotic kin) and regarding them as forming 
a series parallel to the series of fungi, much as the fungal series is 
parallel to the algal. 
188. The thallophyte division, lobeworts (Thallophyta) 
although composed of the humblest members of the vegetable 
kingdom yet contains, as we have seen, some of man’s best 
friends, and also some of his most harmful enemies, a knowl- 
edge of which gained only within recent years has been of 
incalculable benefit to mankind through improving methods 
