THE LICHEN LIFE-CYCLE 1G7 



Types formerly accepted as elementary, with a two-phase cycle 

 (jSTemalion, Scinaia) 1 , are now clearly recognized as examples of 

 deterioration following a state of autogamy, in which the loss 

 of a tetrasporic individual is associated with causal or consequent 

 secondary localization of meiosis at the germination of the zygote. 

 The gametophyte individuals attain fertilization in situ, with a 

 resultant holoparasitic sporophyte-generation, which, it is true, is 

 nourished, protected, and even its spore-output controlled by cysto- 

 carpic and ostiolar mechanism; but it does not produce a hv- 

 menial layer of sporangia and paraphyses, nor even of meiotic 

 sporangia. The entire uninucleate content of each end-ramulus 

 of the cystocarpic plant (carposporophyte) is discharged as a single 

 monospore [carp on pore) ; and it takes a third generation {ictra- 

 sporophyte) to provide the meiotic tetrads. The meiotic unilocular 

 sporangium thus produced has equally constantly one meiotic tetrad 

 with no further mitotic divisions, and so far presents a stage clearly 

 one degree more limited than the normal ascus with its set of eight 

 ascospores. In three fundamental respects, which all appear as stages 

 of progression respectively more advanced than those of modern 

 Ascomycetes, the latter present indications of an algal organization 

 older than that of the modern Florideae, and one of theoretically 

 greater simplicity ; nearer, that is to say, the horizon of such a form 

 as Cutleria. 



On the other hand, biological parallelism in the mechanism of 

 fertilization is at first sight very close; and it is to this that atten- 

 tion has been directed more particularly by advocates of the primary 

 value of sexual relations. The details of Floridean genera indicate 

 with great unanimity a differentiation of ' spermatia,' as the contents 

 of the antheridium are discharged in a uninucleate condition, ' in 

 endochiton ' in the manner of monospores " — i. e. still surrounded 

 by the lining-layer of the mother-cell — a method wholly distinct 

 from the abstriction of the conidial spermatia of Lichens 3 . The 

 carpogonium of the Floridea? is extraordinarily constant as a residual 

 minimum ramulus of an older filamentous type of soma, commonly 

 represented hy a pedicel-region of three cells, with a terminal ' carpo- 

 gonium ' extended as a mucilage-hair 'trichogyne,' and furnished 

 with two nuclei, only one of which is employed as the female gamete. 

 Fertilization follows spermatogamic conjugation, and the diploid 

 zygote-nucleus, in the more progressive types, is left implanted in 

 the plasma of an 'auxiliary' feeding-cell which initiates the next 

 generation. But here, again, there is no homology with the corre- 

 sponding ramuli of the Lichen -procarp and its uniseriate row of cells 

 as so-called ' trichogyne.' The physiological mechanism is analogous, 

 but the details are clearly not homologous ; i. e., the mechanism is 

 devoted to the same end, the general scheme of operation is much 

 the same, and the resultant effect is much the same ; but the 

 details of the corresponding mechanism belong clearly to two quite 



1 Kylin (1916) Berichte, 34, p. 257 : Cloland (1919) Annals Bot., p. 323. 



2 Guignard (1889) Revue Gen. Bot., p. 175. 



3 Yamanouchi (1906) Bot. Gaz. 42, p. 411, describes the spermatium of 

 Polysiphonia as directly abstricted, and the Lichen-type may be equally 

 derivative. 



