b. From polytomous Branching. Another method of scyphus formation occurs in Cl. amaurocrea and a few other species in which the branching is polytomous (several members rising from about the same level). Concrescence of the tissues at the base of these branches produces a scyphus; it is normally closed by a diaphragm that has spread out from the different bases, but frequently there is a perforation due to stretching. These species belong to the Section Perviae.
c. From arrested Growth. In most cases however where the scyphus is open as in Cl. furcata, Cl. squamosa, etc., development of the cup follows on cessation of growth, or on perforation at the summit of the podetium. Round this quiescent portion there rises a circle of minute prominences which carry on the apical development. As they increase in size, the spaces between them are bridged over by lateral growth, and the scyphus thus formed is large or small according to the number of these outgrowths. Apothecia or spermogonia may be produced at their tips, or the vegetative development may continue. Scyphi formed in this manner are also open or “pervious.”
d. Gonidia of the Scyphus. Gonidia are absent in the early stages of scyphus formation when it arises from degeneration of the apical tissues, either fertile or vegetative; but gradually they migrate from the podetium, from the base of young outgrowths, or by furrows at the edge, and so spread over the surface of the cup. Soredia may possibly alight, as Krabbe insists that they do, and may aid in colonizing the naked area. Their presence, however, would only be accidental; they are not essential, and scyphi are formed in many non-sorediate species such as Cl. verticillata. The cortex of the scyphus becomes in the end continuous with that of the podetium and is always similar in type.
e. Species without Scyphi. In species where the whole summit of the podetium is occupied by an apothecium, as in Cl. bellidiflora, no scyphus is formed. There is also an absence of scyphi in podetia that taper to a point. In those podetia the hyphae are parallel to the long axis and remain in connection with the external gonidial layer so that they are unaffected by the central cavity. Instances of tapering growth are also to be found in species that are normally scyphiferous such as Cl. fimbriata subsp. fibula, and Cl. cornuta, as well as in species like Cl. rangiferina that are constantly ascyphous.
The scyphus is considered by Wainio[400] to represent an advanced stage of development in the species or in the individual, and any conditions that act unfavourably on growth, such as excessive dryness, would also hinder the formation of this peculiar lichen structure.
D. Branching of the Podetium
Though branching is a constant feature in many species, regular dichotomy is rare: more often there is an irregular form of polytomy in which one of the members grows more vigorously than the others and branches again, so that a kind of sympodium arises, as in Cl. rangiferina, Cl. sylvatica, etc.
Adventitious branches may also arise from the podetium, owing to some disturbance of the normal growth, some undue exposure to wind or to too great light, or owing to some external injury. They originate from the gonidial tissue in the same way as does the podetium from the primary thallus; the parallel hyphae of the main axis take no part in their development.
In a number of species secondary podetia arise from the centre of the scyphus—constantly in Cl. verticillata and Cl. cervicornis, etc., accidentally or rarely in Cl. foliacea, Cl. pyxidata, Cl. fimbriata, etc. Wainio[401] has stated that they arise when the scyphus is already at an advanced stage of growth and that they are to be regarded as adventitious branches.
The proliferations from the borders of the scyphus are in a different category. They represent the continuity of apical growth, as the edges of the scyphus are but an enlarged apex. These marginal proliferations thus correspond to polytomous branching. In many instances their advance is soon stopped by the formation of an apothecium and they figure more as fruit stalks than as podetial branches.