Fig. 76. Ricasolia amplissima de Not. (Lobaria laciniata Wain.) on oak, reduced. The dark patches are tufts of branching cephalodia (A. Wilson, Photo.).
As cephalodia contain rather dark-coloured, blue-green algae, they are nearly always noticeably darker than the thalli on which they grow, varying from yellowish-red or brown in those of Lecanora gelida to pale-coloured in Lecidea consentiens[456], a darker red in Lecidea panaeola and various shades of green, grey or brown in Stereocaulon, Lobaria (Ricasolia), etc. They form either flat expansions of varying size on the upper surface of the thallus, rounded or wrinkled wart-like growths, or upright branching structures. On the lower surface, where they are not unfrequent, they take the form of small brown nodules or swellings. In a number of species packets of blue-green algae surrounded by hyphae are found embedded in the thallus, either in the pith or immediately under the cortex. They are of the same nature as the superficial excrescences and are also regarded as cephalodia.
B. Classification
Forssell has drawn up a classification of these structures, as follows:
I. Cephalodia vera.
1. Cephalodia epigena (including perigena) developed on the upper outer surface of the thallus, which are tuberculose, lobulate, clavate or branched in form. These are generally corticate structures.
2. Cephalodia hypogena which are developed on the under surface of the thallus; they are termed “thalloid” if they are entirely superficial, and “immersed” when they are enclosed within the tissues. They are non-corticate though surrounded by a weft of hyphae. Forssell further includes here certain placodioid (lobate), granuliform and fruticose forms which develop on the hypothallus of the lichen, and gradually push their way up either through the host thallus, or, as in Lecidea panaeola, between the thalline granules.
Nylander[457] arranged the cephalodia known to him in three groups: (1) Ceph. epigena, (2) Ceph. hypogena and (3) Ceph. endogena. Schneider[458] still more simply and practically describes them as Ectotrophic (external), and Endotrophic (internal).