THE LICHEN LIFE -CYCLE 143 



such simple algal antecedents, it should be now possible to analyse 

 the organization of a saprophytic Ascomycete, and place it in its proper 

 relation to other plant-groups of comparable status and with similar 

 transmigrant origin. It is therefore important to realize that such a 

 complex life-cycle, having attained to fertilization in situ, and a two- 

 phase sequence accurately adjusted to a condition of alternation by 

 cytological mechanism, can only supervene in plants of high-grade 

 specialization in other respects. That is to say, with complex somatic 

 elaboration, as characteristic of fairly large and efficient autotrophic 

 forms, with morphological differentiation of members and anatomical 

 differentiation of tissues, as physiological division of labour attains a per- 

 fection as great (mutatis mutandis) in the life of the individual, as is 

 the part played by the complexities of reproductive mechanism in the 

 life of the race \ In such advanced organism the successful secondary 

 assumption of the heterotrophic habit involves the loss of all peri- 

 pheral photosynthetic tissues, and any mechanism implied by older 

 conditions of surface-nutrition. The soma is reduced to the merest 

 skeleton of ' mechanical hypha3,' as what may be termed a ' skinned 

 alga' ; though possibly long retaining evidence of ramification, older 

 somatic form-factors, or the differentiation of reproductive tracts (cf. 

 Xylana). In the limit it reduces more and more to an amorphous 

 mass of interwoven hyphse (Py roue ma), the ' thallus ' of oldei 

 writers ~, or to the merest weft of a few mycelial filaments, in ultimate 

 stages of parasitic deterioration on, or even within, the active tissues 

 of the foliage-lea ves of higher land-plants (Sjihcerotheea, Edwascns). 

 These and other limiting extremes of the series have attracted con- 

 siderable attention in the past, owing to the unfortunate obsession 

 of land-botanists for confusing the apparently ' simple ' with the 

 ' primitive.' The phyletic value of all such residual heterotrophic 

 organism of the land is considerably discounted once the autotrophic 

 marine origin of all land- vegetation is recognized ; as also the fact that 

 the greater the complexities of the subaerial transmigration, the more 

 abundant and aberrant will be such vestigial forms as the debris 

 of lost races. 



While the deterioration of the somatic region is to be found in 

 every grade, the organization of the reproductive processes may remain 

 comparatively unaffected ; since the latter were originally tending to 

 be the more heterotrophic as more specialized, and so the more 

 dependent on food-material derived elsewhere. Thus Pyronema 

 presents vestigia of what might pass for quite definitely differentiated 

 antheridia and oogonia as ' unilocular gametangia' ; though these now 

 grow closely adjacent, with their mechanism for the discharge of 

 flagellated gametes wholly eliminated. With the latter has also gone 



1 This generalization has been much obscured by land-botanists confined to 

 the consideration of the last and most depauperated algas of fresh water (Goleo- 

 chiete, Vanclierla), in which advanced condition of reproductive mechanism may- 

 be associated with extreme somatic decadence. The same applies to common 

 moulds, as Enrotium, Penicilliivm, and limiting phrases of parasitic habit on 

 foliage -leaves (Sphserotheca, Exoascus, Uredinete, etc.). 



2 Schleideu (Eng. Trans. 1849), p. Ia7 ; Sachs (1874), Eng, Trans. 1882, 

 p, 231. 



