FUNGI. LICHENS. 119 



gonidial layer are increased in number. It is plain then that in Usnea barbata the 

 growth in length, the growth in thickness, and the internal differentiation of the tissue 

 must be wholly put to the account of the hyphae, and that the gonidia behave as a 

 foreign addition to the hyphal tissue. In accordance with this the formation of new 

 branches originates with the hyphae and not with the gonidia. The branching may be 

 dichotomous, and if so, the apical members of the hyphae incline towards two points 

 lying near each other and then grow on in corresponding directions, so that the two 

 equal forks form an acute angle. Adventitious lateral branches arise behind the extremity 

 of the thallus by the fibres of the rind forming a new apex and growing in an outward 

 direction ; there are gonidia also to be found behind the apex of the new branch ; the 

 base of the branch sends fibres of the tissue beneath the rind and an axile bundle into 

 the mother-branch, and thus the corresponding forms of the tissue of the two are 

 united together. The growth of Usnea may be compared in its essential points with 

 the growth of the stroma of Xylaria ; the gonidia are here a subordinate element in 

 the formation of the whole ; and the strands of the Rhizomorphae, which will be 

 described later on under the Basidiomycetes, supply, as Brefeld has pointed out, an 

 instance of still closer analogy amongst the Fungi. In many crustaceous Lichens the 

 thallus has as a rule no certain outline ; there is no such definite external form as in 

 the cases which we have been considering ; the thallus appears to be composed of 

 groups of gonidia somewhat irregularly disposed and of hyphae growing among and 

 between them. In others however, such as Sporastatia morio, Rhizocarpon subcentri- 

 cum, Aspirilia calcarea, the thallus forms lobed disks, which expand by centrifugal 

 growth at the margin ; the growing margin is composed of hyphal tissue only, and 

 groups of gonidia appear first at separate spots further inwards, that is, nearer the 

 centre, and spread by degrees. The rind-tissue shows indentations at the cir- 

 cumference of the spots where the gonidia are collected, and isolated scale-like bits of 

 a true lichen-thallus are in consequence formed upon a fibrous substratum known as 

 the hypothallus^-. 



Some of the Lichens which live on the bark of trees, the GRAPHIDEAE especially, 

 exhibit peculiarities, which Frank 2 has recently investigated. They pass through two 

 different states in their existence ; .' one in which they are without gonidia and consist 

 only of hyphae, and one in which they are typical Lichens composed of hyphae and 

 gonidia.' In the first state the hyphae of Arthonia vulgaris and Graphis scripta form 

 inside the outermost corky layer of the periderm of the trees which they inhabit a toler- 

 ably close and coherent felt of extremely delicate hyphae, which spread irregularly and 

 in every direction among the cells of the tissue, forming a homogeneous substratum 

 and causing certain changes in the appearance of the periderm. This layer of hyphal 

 tissue grows centrifugally in breadth, and eventually forms the marginal zone of the 

 thallus. The thallus is produced by the intrusion into the hyphal tissue of gonidia 

 belonging to the algal genus Chroolepus, a filamentous Alga very nearly related to 

 Cladophora, and whose cells are usually coloured with a red oil. It is only after 

 the entrance of the gonidia that a fructification is formed. But all the Lichens 

 that live in cortical tissue, which Frank calls * hypophloeodic Lichens,' are not of 

 this kind. Lecanora pallida makes its way into the cortex with the first gonidia that 

 are attacked by the hyphae, and obtains the rest of the gonidia by multiplication 

 of the original ones. The genus Arthonia above mentioned has also species which 

 have no gonidia ; while A. vulgaris has gonidia, A . punctiformis is without them and is 

 therefore a true Fungus. 



That the Lichens are simply Ascomycetous Fungi which live in association with 

 Algae may be considered to be proved by the facts which have now been recounted. 



1 See Schwendener in Flora, 1865, Nr. 26. 



2 Frank, Ueber d. biolog. Verhaltnisse d. Thallus einigen Krustenflechten in Cohn's Beitr. z. Biol. 

 Pflanzen, II. p. 213. 



