104 



MORPHOLOGY 



sometimes as a continuous ring which may project irregularly into the pith 

 (./?. calicaris) ; more frequently it is in the form of strands or bundles which 

 alternate with the groups of gonidia (R. siliquosa, R. Curnowii, etc.). In 

 R.fraxinea these strands may be scarcely discernible in young fronds, though 

 sometimes already well developed near the tip's. Occasionally isolated strands 

 of fibres appear in the pith (R. Curnowii}^ or the sclerotic projections may 

 even stretch across the pith to the other side (R. strepsilis) (Fig. 75 B). 



In the Cladoniae support along with flexibility is secured to the upright 

 podetium by the parallel closely packed hyphae that form round the 

 hollow cylinder a band called the "chondroid" layer from its cartilage-like 

 consistency. 



b. CHONDROID AXIS. The central medullary tissue in Ramalina is, with 

 few exceptions, a loose arachnoid structure; often the fronds are almost 

 hollow. In one species of Usnea, U. Taylori, found in polar regions, there 

 is a similar loose though very circumscribed medullary and gonidial tissue 

 in the centre of the somewhat cylindrical thallus, and a wide band of sclerotic 

 fibres towards the cortex. 



; 



Fig. 63 A. A, Usnea barbataWeb. Longitudinal section of filament with young adventitious 

 branch. , chondroid axis; />, gonidial tissue; c, cortex, x 100 (after Schwendener). B, U. 

 longissima Ach. Hyphae from central axis x 525 (after Schulte). 



In all other species of Usnea the medulla itself is transformed into a 

 strong central strand of long-celled thick-walled hyphae closely knit together 

 by frequent anastomoses (Fig. 63 A). This central strand of the Usneas is 

 known as the "chondroid axis." A narrow band of loose air-containing 

 hyphae and a gonidial zone lie round the central axis between it and the 

 outer cortex (Fig. 63 A, b). At the extreme apex, the external cortical hyphae 

 grow in a direction parallel with the long axis of the plant, but further back, 

 they branch out at right angles and become swollen and mostly "decom- 

 posed " as in the cortex of Ramalina. 



