320 



THALLOPHYTES. 



two, a foliaceous expansion of small size being first fornned, the cup-shaped or fruticosely- 

 branched thallus afterwards rising from this. 



Fig. 213. — A Usnea barbata, a fruticose Lichen (natural size) ; B SHcta pulmojtacea, a foliaceous Lichen (natural size) 

 seen from beneath ; a apothecia.ythe attaching disc of ^, by which the Lichen becomes attached to the bark of a tree. 



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 consistence, is tough, elastic, 



^ ^^^ _^ - ^_ and flexible ; but a large number of 



w^^^m. 



Fig. 214. — Transverse section through the foliaceous thallus of Sticta 

 fidiginosa (X 500) ; o cortical or epidermal layer of the upper side ; u of 

 the under side ; r r rhizines or attaching fibres, springing from the 

 epidermal layer and' therefore trichomes ; m the medullary layer, the 

 hyphse of which are seen cut, some transversely, some longitudinally. 

 The upper and under cortical layers also consist of hyph?e, which how- 

 ever are much thicker, consist of shorter cells, and are united without 

 interstices, forming a pseudo-parenchyma ; g the gonidia (tlieir light- 

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

 velope encloses several gonidia produced by division. 



genera, which are remarkable also 

 in other ways, are slimy and gela- 

 tinous 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 Collema, Fig. 212. 

 The disposition of the gonidia and 

 hyphae in a thallus may be such that 

 these two structures appear about 

 equally mingled (as in Fig. 215), 

 and the thallus is in this case called 

 homoiomerous ; or the gonidia are 

 crowded into one layer (as in Fig. 

 214), by which the hyphal tissue is 

 at the same time separated accord- 

 ing 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. 214 and 



217). 



The mode of growth, branching, 

 and external structure of the Lichen- 



