THE THALLOPHYTE DIVISION 



509 



much economic importance. They ma}' be defined as algo- 

 fungal air-plants. Although made up of plants which belong 

 to chfferent classes of AlgiB 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. 



Fig. 337. — Mushroom-lichen (Cora pavona, Mushroom-lichen Family, 

 Coraccw). A, top view of fruit-t^ody, natural size. B, under side 

 showing hymenium (hyrn). (Strasburgor.) — On trees in tlie tropics. 



Their dual nature, indeed, doulsles the difficultj' 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 effecti^^ely by using names like Ascolichenes 

 and Basidiolichenes (suggestive of relationship between the lichen- 

 fungi and their non-sjmibiotic 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 knowd- 

 edgc of which gained only within recent years has been of 

 incalculable benefit to mankind through improving methods 



