I04 



MORPHOLOGY 



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

 (Ji. 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 tips. 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 barbata Web. Longitudinal section of filament with young adventitious 

 branch, a, chondroid axis; b, gonidial tissue; c, cortex, x loo (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. 



