198 



MECHANICAL SYSTEM 



more or less centrally in accordance with the tension-resisting character 

 of those parts of the thallus. 



Among Fungi thick-walled mechanical hyphae are on the whole rare. 

 The subterranean rhizomorphs found in the Phalloideae and Lycopek- 

 daceae, and in some species of Agaricas, consist of a relatively loose 

 "cortex" composed of thin-walled hyphae and enclosing a core of thick- 

 walled gelatinous filaments, which are evidently responsible for the inex- 

 tensibility of the whole structure ; it is doubtful whether this core may 

 not in addition serve for translocation, and perhaps also for other purposes. 

 Experiments performed by the author indicate that the " medullary 

 strand " in the branches of the fruticose lichen Usnea barbata likewise 

 represents a tension-resisting device (Fig. 71 a). In this instance the 



B 



Fig. 71. 



A. T.S. through a pendulous branch of Usnea barbata (explanation in text). H. T.S. 

 through the hollow inflexible podetium of Cladonia filiformis . x 40. A, dermal tissue ; 

 g, gonidial layer; I, loose plectenchyma representing the ventilating system; m, 

 mechanical system. 



central bundle of thick-walled hyphae resembles, as regards its strength 

 and elasticity, such a material as india-rubber, rather than bast or collen- 

 chyma. For one thing it is extraordinarily extensible. In the fresh 

 and fully imbibed condition, the medullary strand in a branch "5 to 

 1 mm. in diameter, pertaining to a pendulous variety of this lichen, 

 could be stretched to more than twice its original length (an elongation 

 of 100-110 per cent.) without breaking. In the case of a more 

 shrubby and thick-set variety, branches of the same thickness were 

 found to break when stretched to the extent of 00-70 per cent. In 

 both instances the medullary strands remained perfectly elastic after 

 undergoing an extension of 20 per cent. The much smaller extensi- 

 bility of the cortical and dermal tissues betrayed itself, in these 

 experiments, by the appearance of numerous wide transverse fissures in 

 the outer part of the branch. The breaking-weight of the medullary 

 strands amounts on an average to 1*7 kg. per sq. mm., and is thus 

 approximately the same as that of the foliar epidermis of Allium 

 Porrum, which is estimated by Weinzierl at l"5-l - 8 kg. per sq. mm. 



