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MORPHOLOGY 



700 



the plant on account of the supposed resemblance of the dotted thallus to 

 the infantile ailment of " thrush." Babikoff 1 has published an account of 

 the formation and development of these Peltidea cephalodia. He determined 

 the algae contained in them to be Nostoc by isolating and growing them on 

 moist sterilized soil. He observed that the smaller, and presumably younger, 

 excrescences were near the edges of the lobes. The cortical cells in that 

 position grow out into fine septate hairs that are really the ends of growing 

 hyphae. Among the hairs were scattered minute colonies of Nostoc cells 

 lying loose or so closely adhering to the hairs as to be undetachable (Fig. 



78 A). In older stages the hairs, evi- 

 dently stimulated by contact with the 

 Nostoc, had increased in size and sent 

 out branches, some of which penetrated 

 the gelatinous algal colony; others, 

 spreading over its surface, gradually 

 formed a cortex continuous with that 

 of the thallus. The alga also increased, 

 and the structure assumed a rounded or 

 lentiform shape. The thalline cortex 

 immediately below broke down, and 

 the underlying gonidial zone almost 

 wholly died off and became absorbed. 

 The hyphae of the cephalodium had 

 meanwhile penetrated downwards as root-like filaments, those of the thallus 

 growing upwards into the new overlying tissue (Fig. 78 B). The foreign 

 alga has been described as parasitic, as it draws from the lichen hyphae the 

 necessary inorganic food material; but it might equally well be considered 

 as a captive pressed into the service of the lichen to aid in the work of assi- 

 milation or as a willing associate giving and receiving mutual benefit. 



Th. M. Fries 2 had previously described the development of the cephalodia 

 in Stereocaulon but failed to find the earliest stages. He concluded from his 

 observations that parasitic algae were common in the cortical layer of the 

 lichens, but only rarely formed the." monstrous growths " called cephalodia. 

 b. ENDOTROPHIC. Winter 3 examined the later stages of internal cepha- 

 lodine formation in a species of Sticta. The alga, probably a species of 

 Rivularia, which gives origin to the cephalodia, may be situated immediately 

 below the upper cortex, in the medullary layer close to the gonidial zone, 

 or between the pith and the under cortex. The protuberance caused by the 

 increasing tissue, which also contains the invading alga, arises accordingly 

 either on the upper or the lower surface. In some cases it was found that 

 the normal gonidial layer had been pushed up by the protruding cephalodium 

 1 Babikoff 1878. 2 Th. M. Fries 1866. 3 Winter 1877. 



Fig. 78 A. Hairs of Peltigera aphthosa Willd. 

 associated with Nostoc colony much mag- 

 nified (after Babikoff). 



