114 MORPHOLOGY 



2. RADIATE OR SECONDARY THALLUS 



A. Origin. OF the Podetium 



The upright podetium, as described by Wainio' and by Krabbe'', is a 

 secondary product of the basal granule or squamule. It is developed from 

 the hyphae of the gonidial zone, generally where a crack has occurred in the 

 cortex and rather close to the base or more rarely on or near the edge of 

 the squamule {CI. verticillata, etc.). At these areas, certain meristematic 

 gonidial hyphae increase and unite to form a strand of filaments below the 

 upper cortex but above the gonidial layer, the latter remaining for a time 

 undisturbed as to the arrangement of the algal cells. 



This initial tissue — the primordium of the podetium — continues to grow 

 not only in width but in length : the basal portion grows downwards and 

 at length displaces the gonidial zone, while the upper part as a compact 

 cylinder forces its way through the cortex above, the cortical tissue, however, 

 taking no part in its formation ; as it advances, the edges of the gonidial 

 and cortical zones bend upwards and form a sheath distinguishable for some 

 time round the base of the emerging podetium. 



Even when the primary horizontal thallus is merely crustaceous, the 

 podetia take origin similarly from a subcortical weft of hyphae in an areola 

 or granule. 



B. Structure of the Podetium 



a. General structure. In the early stages of development the 

 podetium is solid throughout, two layers of tissue being discernible — the 

 hyphae forming the centre of the cylinder being thick-walled and closely 

 compacted, and the hyphae on the exterior loosely branching with numerous 

 air-spaces between the filaments. 



In all species, with the exception of CI. solida, which remains solid during 

 the life of the plant, a central cavity arises while the podetium is still quite 

 short (about i to 1-5 mm. in CI. pyxidata and CI. degenerans). The first 

 indication of the opening is a narrow split in the internal cylinder, due to 

 the difference in growth tension between the more free and rapid increase 

 of the external medullary layer and the slower elongation of the chondroid 

 tissue at the centre. The cavity gradually widens and becomes more com- 

 pletely tubular with the upward growth of the podetium ; it is lined by 

 the chondroid sclerotic band which supports the whole structure (Fig. 67). 



b. Gonidial tissue. In most species of Cladoniaceae, a layer of goni- 

 dial tissue forms a more or less continuous outer covering of the podetium, 



1 Wainio 1880. 2 Krabbe 1891. 



