320 
THALLOPHYTES. 
two, a foliaceous expansion of small size being first formed, the cup-shaped or fruticosely- 
branched thallus afterwards rising from this. 
Fig. 213.—^ Usnca barbata, a fruticose Lichen (natural size) ; B Sticta pulmonacea, a foliaceous Lichen (natural size) 
seen from beneath ; a apothecia.y the 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 
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 theLichen- 
FlG. 214.— Transverse section through the foliaceous thallus oi Sticta 
ftdiginosa (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 trichomas; m the medullary layer, the 
hyphte of which are seen cut, some transversely, some longitudinally. 
The upper and under cortical layers also consist of hyphse, which how- 
ever are much thicker, consist of shorter cells, and are united without 
interstices, forming a pseudo-parenchyma; g- the gonidia (their light- 
green masses of protoplasm are coloured dark) ; each gelatinous en- 
velope encloses several gonidia produced by division. 
