n8 



FIRST GROUP. THALLQPHYTES. 



towards the new apical cell as they have been shown to behave towards the apical cell 

 of the main branch. Usnea barbata, a fruticose Lichen, forms a much-branched 

 shrub-like thallus, resembling that of Ephebe pubescens; in this case also the 

 branches of the thallus elongate by apical growth (Fig. 79, A] ; but this is not 

 effected by means of the gonidia as in Ephebe, nor usually by a single cell, but the hyphae, 

 which run parallel to each other in the branch, and incline towards one another at 

 its extremity, elongate individually by apical growth of the terminal member, and thus 

 co-operate in bringing about longitudinal growth at the apex of the branch ; this growth 

 is supplemented by an intercalary growth further behind due to intercalary elongation 

 of the hyphae and the interpolation of branches of the hyphae in different directions. 



FIG. 78. A branch of the 

 thallus oiEphebepubescenStTQ.'&gn. 

 500 times ; see the text. 



FIG. 79. Usnea barbata. A optical longitudinal section 01 

 a slender branch, softened in potash solution. B transverse sec- 

 tion of an older thallus-stem with the basal portion of an adven- 

 titious (soredial) branch sa, magn. 300 times ; .< apex of the 

 branch, r the rind, x the axial medullary bundle, m the loose 

 medullary tissue, q the gonidial layer. 



The hyphae lie so close together that they form a compact mass without interstices ; 

 but at some distance behind the apex of the branch the hyphal tissue is differentiated 

 into a dense rind formed of fibres interwoven on all sides, an axile longitudinal bundle 

 of closely packed filaments, and between them a layer of looser texture with air- 

 conducting interstices. At the place where this differentiation of the hyphal tissue begins 

 behind the apex, there the layer of gonidia terminates ; this layer is composed of small 

 roundish green cells, which, as they multiply by division, form small groups ; these 

 groups are themselves disposed in a mantle-like layer lying between the rind and the 

 tissue beneath (see the transverse section B in Fig. 79). Single gonidia only lie 

 behind the growing apex of the branch, and by their subsequent division the cells in the 



