Lichens

by Annie Lorrain Smith

This secondary thallus originates in two different ways: (1) the primary granule may grow upward, the whole of the tissues taking part in the new development; or (2) the origin may be endogenous and proceed from the hyphae only of the gonidial zone: these push upwards in a compact fascicle, as in the apothecial development of Lecidea, but instead of spreading outward on reaching the surface, they continue to grow in a vertical direction and form the podetium. In origin this is an apothecial stalk, but generally it is clothed with gonidial tissue. The gonidia may travel upwards from the base or they may possibly be wind borne from the open. The podetium thus takes on an assimilative function and is a secondary thallus.

The same type of apothecium is common to all the genera; the spores are colourless and mostly simple, but there are also changes in form and septation not commensurate with thalline advance, as has been already noted. Thus in Gomphillus, with primitive thallus and podetium, the spores are long and narrow with about 100 divisions.

1. Origin of Cladonia. There is no difficulty in deriving Cladoniaceae from Lecidea, or, more exactly, from some crustaceous species of the section Biatora in which the apothecia—as in Cladoniaceae—are waxy and more or less light-coloured and without a thalline margin. In only a very few isolated instances has a thalline margin grown round the Cladonia fruit.

There are ten genera included in the Cladoniaceae, of which five are British. Considerable study has been devoted to the elucidation of developmental problems within the family by various workers, more especially in the large and varied genus Cladonia which is complicated by the presence of the two thalli. The family is monophyletic in origin, though many subordinate phyla appear later.

2. Evolution of the Primary Thallus. At the base of the series we find here also an elementary granular thallus which appears in some species of most of the genera. In Gomphillus, a monospecific British genus, the granules have coalesced into a continuous mucilaginous membrane. In Baeomyces, though mostly crustaceous, there is an advance to the squamulose type in B. placophyllus, and in two Brazilian species described by Wainio, one of which, owing to the form of the fronds, has been placed in a separate genus Heteromyces. The primary thallus becomes almost foliose also in Gymnoderma coccocarpum from the Himalayas, with dorsiventral stratose arrangement of the tissues, but without rhizinae. The greatest diversity is however to be found in Cladonia where granular, squamulose and almost foliose thalli occur. The various tissue formations have already been described[1007].

3. Evolution of the Secondary Thallus. Most of the interest centres round the development and function of the podetium. In several genera the primordium is homologous with that of an apothecium; its elongation to an apothecial stalk is associated with delayed fructification, and though it has taken on the function of the vegetative thallus, the purpose of elongation has doubtless been to secure good light conditions for the fruit, and to facilitate a wide distribution of spores: therefore, not only in development but in function, its chief importance though now assimilative was originally reproductive. The vegetative development of the podetium is correlated with the reduction of the primary thallus which in many species bears little relation in size or persistence to the structure produced from it, as, for instance, in Cladonia rangiferina where the ground thallus is of the scantiest and very soon disappears, while the podetial thallus continues to grow indefinitely and to considerable size.

4. Course of Podetial Development. In Baeomyces the podetial primordium is wholly endogenous in some species, but in others the outer cortical layer of the primary thallus as well as the gonidial hyphae take part in the formation of the new structure which, in that case, is simply a vertical extension of the primary granule. This type of podetium—called by Wainio[1008] a pseudopodetium—also recurs in Pilophorus and in Stereocaulon. To emphasize the distinction of origin it has been proposed to classify these two latter genera in a separate family, but in that case it would be necessary to break up the genus Baeomyces. We may assume that the endogenous origin of the “apothecial stalk” is the more primitive, as it occurs in the most primitive lecideine lichens, whereas a vertical thallus is always an advanced stage of vegetative development.

Podetia are essentially secondary structures, and they are associated both with crustaceous and squamulose primary thalli. If monophyletic in origin their development must have taken place while the primary thallus was still in the crustaceous stage, and the inherited tendency to form podetia must then have persisted through the change to the squamulose type. In species such as Cl. caespiticia the presence of rudimentary podetia along with large squamules suggests a polyphyletic origin, but Wainio’s[1008] opinion is that such instances may show retrogression from an advanced podetial form, and that the evidence inclines to the monophyletic view of their origin.

The hollow centre of the podetium arises in the course of development and is common to nearly all advanced stages of growth. There are however some exceptions: in Glossodium aversum, a soil lichen from New Granada, and the only representative of the genus, a simple or rarely forked stalk about 2 cm. in height rises from a granular or minutely squamulose thallus. The apothecium occupies one side of the flattened and somewhat wider apex. There is no external cortex and the central tissue is of loose hyphae. In Thysanothecium Hookeri, also a monotypic genus from Australia, the podetia are about the same height, but, though round at the base, they broaden upwards into a leaf-like expansion. The central tissue below is of loose hyphae, but compact strands occur above, where the apothecium spreads over the upper side. The under surface is sterile and is traversed by nerve-like strands of hyphae.

5. Variation in Cladonia. It is in this genus that most variation is to be found. Characters of importance and persistence have arisen by which secondary phyla may be traced within the genus: these are mainly (1) the relative development of the horizontal and vertical structures, (2) formation of the scyphus and branching of the podetium, with (3) differences in colour both in the vegetative thallus and in the apothecia.