26 CONSTITUENTS OF THE LICHEN THALLUS 



the hyphal origin of the gonidia: not only of Cystococcus but also of Trente- 

 pohlia, Stigonema and Nostoc. In the case of Cystococcus, the gonidium, he 

 says, arises by the swelling of the terminal cell of the hypha to a globose 

 form, and by the gradual transformation of the contents to a chlrophyll- 

 green colour, with power of assimilation. In the case of filamentous gonidia 

 such as Trentepohlia, the hyphal cells destined to become gonidia are 

 intercalary. In Peltigera the cells of the meristematic plectenchyma become 

 transformed to blue-green Nostoc cells. 



A study was also made by him of the formation of cephalodia 1 , the 

 gonidia of which differ from those of the " host" thallus. In Peltigera aphthosa 

 he claims to have traced the development of these bodies to the branching 

 and mingling of the external hairs which, in the end, form a ball of inter- 

 woven hyphae. The central cells of the ball are then gradually differentiated 

 into Nostoc cells, which increase to form the familiar chains. Elfving allows 

 that the gonidia mainly increase by division within the thallus, and that they 

 also may escape and live as free organisms. His views are unsupported by 

 direct culture experiments which are the real proof of the composite nature 

 of the thallus. 



E. MlCROGONIDIA 



Another attempt to establish a genetic origin for lichen gonidia was made 

 by Minks 2 . He had found in his examination of Leptogium myochroum that 

 the protoplasmic contents of the hyphae broke up into a regular series of 

 globular corpuscles which had a greenish appearance. These minute bodies, 

 called by him microgonidia, were, he states, at first few in number, but 

 gradually they increased and were eventually set free by the mucilaginous 

 degeneration of the cell wall. As free thalline gonidia, they increased in 

 size and rapidly multiplied by division. Minks was at first enthusiastically 

 supported by Miiller* who had found from his own observations that micro- 

 gonidia might be present ip any of the lichen hyphae and in any part of 

 the thallus, even in the germinating tube of the lichen spore, and was in that 

 case most easily seen when the spores germinated within the ascus. He 

 argued that as spores originated within the ascus, so microgonidia were 

 developed within the hyphae. Minks's theories were however not generally 

 accepted and were at last wholly discredited by Zukal 4 who was able to 

 prove that the greenish bodies were contracted portions of protoplasm in 

 hyphae that suffered from a lowered supply of moisture, the green colour 

 not being due to any colouring substance, but to light effect on the pro- 

 teins an outcome of special conditions in the vegetative life of the plant. 

 Darbishire 5 criticized Minks's whole work with great care and he has arrived 

 at the conclusion that the microgonidium may be dismissed as a totally 

 mistaken conception. 



1 See p. 133. 2 Minks 1878 and 1879. 3 Miiller 1878 and 1884. 4 Zukal 1884. 5 Darbishire 1895!. 



