292 PHYLOGENY 



stereocauloides, the thallus is described by Wainio' as consisting of minute 

 clavate stall<s of interwoven thick-walled hyphae, with gelatinous algae, like 

 Gloeocapsa, interspersed in groups, though with a tendency to congregate 

 towards the outer surface. 



The highest development along this line of advance is to be found in the 

 Gyrophoraceae, a family of lichens with a varied foliose character and dark 

 lecideine apothecia. The thallus may be monophyllous and of fairly large 

 dimensions or polyphyllous; it is mostly anchored by a central stout hold- 

 fast and both surfaces are thickly corticate with a layer of plectenchyma; 

 the under surface is mostly bare, but may be densely covered with rhizina- 

 like strands of dark hyphae. They are all northern species and rock-dwellers 

 exposed to severe extremes of illumination and temperature, but well 

 protected by the thick cortex and the dark colouration common to them all. 



cc. Cladoniaceae. This last phylum of Lecideales is the most interesting 

 as it is the most complicated. It possesses a primary, generally sterile, 

 thallus which is dorsiventral and crustaceous, squamulose or in some in- 

 stances almost foHaceous, along with a secondary thallus of upright radiate 

 structure and of very varied form, known as the podetium which bears at 

 the summit the fertile organs. 



A double thallus has been suggested in the spreading base, containing 

 gonidia, of some radiate lichens such as. Roccella, but the upright portion 

 of such lichens, though analogous, is not homologous with that of 

 Cladoniaceae. 



The algal cells of the family belong to the Protococcaceae. Blue-green 

 algae are associated in the cephalodia of Pilophorus and Stereocaulon. 

 The primary thallus is a feature of all the members, though sometimes very 

 slight and very short-lived, as in Stereocaulon or in the section Cladina of 

 the genus Cladonia. Where the primary thallus is most largely developed, 

 the secondary (the podetium) is less prominent. 



This secondary thallus originates in two different ways: (i) 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 



■* Wainio 1890. 



