In a remarkable paper on The Symbiosis of Lichens[1326], Dr A. Henry Church has presented a new and striking view of the origin and development of lichens: he has sought to link them up with other classes of vegetation that, in the great transmigration, passed from sea to land. As we know from his Thalassiophyta[1327] and the subaerial transmigration, he holds that primeval algae of advanced form and structure were left exposed on dry land by the gradually receding waters, and those that successfully adapted themselves to the changed conditions formed the basis of the land flora. A certain number of the algae lost their surface tissues containing chlorophyll and they had perforce to secure from other organic sources the necessary carbohydrates: they adopted a heterotrophic existence as saprophytic or parasitic fungi. Fungi are a backward race (deteriorated according to Dr Church) as regards their soma, but in number, distribution and variety of spore-production, they are eminently successful plants.

Lichens are similarly regarded by Dr Church as derived from stranded contemporaneous types of marine algae—crustaceous, foliose and fruticose, that had also lost their chlorophyll, but by taking into association green algal units of a lower grade they established a vicarious photosynthesis. But, to quote his own words[1328], “as the alga-lichen-fungus left the sea, so it remained: it might deteriorate, but it certainly never advanced, once the sea factors which produced it were eliminated, it simply stopped along these lines.”

And again[1329]: “Lichens thus present an interesting case of an algal race deteriorating along the lines of a heterotrophic existence, yet arrested, as it were, on the somatic down-grade, by the adoption of intrusive algal units of lower degree to subserve photosynthesis (much in the manner of the marine worm Convoluta). Thus arrested, they have been enabled to retain more definite expression of more deeply inherent factors of sea-weed habit and construction than any other race of fungi; though closely paralleled by such types as Xylaria (Ascomycete) and Clavaria (Basidiomycete), which have followed the full fungus progression as holosaprophytic on decaying plant residues.”

Dr Church’s theory is of vivid interest and might be convincing were there no possibility and no proof of advance within the symbiotic plant, but in numbers of crustaceous thalli, there is evident, by normal or abnormal[1330] development, the first advance to the formation of rudimentary squamules, a condition diagnosed as subsquamulose. “Deterioration” of the lichen plant—when it occurs owing to unfavourable conditions—is a reversion to the leprose early stage of the association; there is no evidence of reversion from fruticose or foliose to squamulose. A glance at the table of lichen phyla[1331] shows progression again and again from the crustaceous forms onwards. In such a phylum as Physciaceae (with colourless polarilocular spores) there is a clear example of a closely connected series; the different types of thallus—crustaceous, squamulose, foliose and fruticose—are all represented and form a natural sequence, being well delimited by the unusual form of the spore and by the presence of parietin in thallus or apothecium.

That there has been development seems absolutely certain, and that along the lines sketched in the chapter on phylogeny. Progress has been mainly in the thallus, but there has also been change and advance in the reproductive organs, more especially in the spores which in several families reach a size and septation unparalleled in fungi. That association with green algal cells stimulated the fungus to new development is the view taken of the lichen plant and emphasized in the present volume. But it seems more in accordance with the polyphyletic origin and recurring parallel development in the phyla that association began at the elementary crustaceous stage, and that the lichen soma was gradually evolved within what is after all a very limited and simple structure.