ORIGIN OF LICHEN STRUCTURES 69 



In the Collemaceae, the gonidial cells of which are species of Nostoc 

 (Fig. 2), there appears a more developed thallus; but in general, symbiosis 

 in Collema has wrought the minimum of change in the habit of the alga, 

 hence the indecision of the earlier botanists as to the identification and 

 classification of Nostoc and Collema. Though in many of the species of the 

 genus Collema no definite tissue is formed, yet, under the influence of 

 symbiosis, the plants become moulded into variously shaped lobes which 

 are specifically constant. In some species there is an advance towards 

 more elaboration of form in the protective tissues of the apothecia, a layer 

 of thin-walled plectenchyma being occasionally formed beneath or around 

 the fruit as in Collema granuliferum. 



In all these lichens, it is only the thallus that can be considered as 

 V primitive: the fruit is a more or less open apothecium — more rarely a peri- 

 thecium — with a fuUj' developed hymenium. Frequently it is provided with 

 a protective thalline margin. 



b. Exogenous Thallus. In this group, composed almost exclusively 

 of heteromerous lichens, Zukal includes all those in which the fungus takes 

 the lead in thalline development. He counts as such Leptogium, a genus 

 closely allied to Collema but with more membranous lobes, in which the 

 short terminal cells of the hyphae have united to form a continuous cortex. 

 A higher development, therefore, becomes at once apparent, though in some 

 genera, as in Coenogonium, the alga still predominates, while the simplest 

 forms may be merely a scanty weft of filaments associated with groups of 

 algal cells. Such a thallus is characteristic of the Ectolechiaceae, and some 

 Gyalectaceae, etc., which have, indeed, been described by Zahlbruckner^ 

 as homoiomerous though their gonidia belong to the non-gelatinous 

 Chlorophyceae. 



Heteromerous lichens have been arranged by Hue^ according to their 

 general structure in three great series : 



1. Stratosae. Crustaceous, squamulose and foliose lichens with a 

 dorsiventral thallus. 



2. Radiatae. Fruticose, shrubby or filamentous lichens with a strap- 

 shaped or cylindrical thallus of radiate structure. 



3. Stratosae-Radiatae. Primary dorsiventral thallus, either crustaceous 

 or squamulose, with a secondary upright thallus of radiate structure called 

 the podetium (Cladoniaceae). 



1 Zahlbruckner 1907. ^ Hue 1899. 



