FUNGI. 



26 



A third form of the Lichen-thallus, also united with the previous one by transitional 

 forms, is shown in the Fruticose Lichens, which are attached only at one spot and 

 with a narrow base, and rise from it .,^ .^ _ , 



in the form of small much-branched 

 shrubs. The branches of the thallus 

 are either flat and ligulate, like the 

 lobes of many foliaceous Lichens, 

 or slender and cylindrical (Fig. 187). 

 In Cladonia and Stcreocaulon we 

 have not so much a transition from 

 the foliaceous to the fruticose thallus 

 as a combination of the two, a folia- 

 ceous expansion of small size being 

 first formed, the cup-shaped or fruti- 

 cosely- branched thallus afterwards 

 rising from this. 



The thallus of Lichens can be dried 

 so as to be pulverised without losing its 

 vitality. When saturated with water 

 it has generally a leathery consist- 

 ence, is tough, elastic, and flexible ; 

 but a large number of genera, which 

 are remarkable also in other ways, are 

 slimy and gelatinous in this condition. 

 These Gelatinous Lichens, as they 

 are termed, form cushion-like masses 

 with an undulated surface, and in their 

 growth are sometimes more like the 

 fruticose, sometimes more like the 

 foliaceous Lichens. A typical form 

 is shown in Coilema, Fig. 186. 



The disposition of the gonidia and hyphae in a thallus may be such that these two 

 structures appear about equally mingled (as in Fig. 189), and the thallus is in this case 



Fig. 188. — Transverse section throisgh the foliaceous thallus oi Stutn/u- 

 lijrinosa (X500) ; o cortical or epidermal layer of the upper side ; n of the 

 under side ; rr rhizines or attaching fibres, springing from the epidermal 

 layer and therefore trichonies ; 771 the medullary layer, the hyphce of 

 which are seen cut, sonic transversely, some longitudinally. The upper 

 and under cortical layers also consist of hyphre, which however are 

 nuich thicker, coiisist of shorter cells, and are united without inter- 

 stices, forming a pseudo-parenchyma ; g the gonidia (their light-green 

 masses of protoplasm are coloured dark) ; each gelatinous envelope en- 

 closes several gonidia produced by division. 



FIG. 1S9.— Vertical section of the gelatinous thallus of LeJ>to,cM»t scotinum (X500) ; an epidermal layer clothes the interior 

 tissue, which consists mainly of shapeless and colourless jelly in which lie the coiled chains of gonidia; some of the larger 

 cells of the chains are left white; between them run the fine hyphae. 



called Homoomerous; or the gonidia are crowded into one layer (as in Fig. 191), by 

 which the hyphal tissue is at the same time separated according to circumstances 

 into an outer and inner or an upper and under layer ; the thallus-tissue is then stratified, 

 and such Lichens are termed Heteromerous (Figs. 188 and 191). 



