STRATOSE THALLUS 



n 



cell-walls of the filaments become much thicker and though, in some species, 

 they remain colourless, in others they become dark-coloured, all except the 

 extreme tips, owing to the presence of lichen pigments — a provision, ZukaU 

 considers, to protect them against the ravages of insects, etc. The pro- 

 thallic filaments adhere closely to the substratum and the branching 

 becomes gradually more dendroid in form, though sometimes hyphae are 

 united into strands, or even form a kind of plectenchymatous tissue. This 

 purely hyphal stage may persist for long 

 periods without much change. In time 

 there may be a fortuitous encounter with 

 the algae (Fig. 38 A) which become the 

 gonidia of the plant. Either these have 

 been already established on the substra- 

 tum as free-growing organisms, or, as 

 accidentally conveyed, they alight on the 

 prothallus. The contact between alga 

 and hypha excites both to active growth 

 and to cell-division; and the rapidly 

 multiplying gonidia are as speedily sur- 

 rounded by the vigorously growing hyphal 

 filaments. 



Schwendener^ has thus described the 

 origin and further development of pro- 

 thallus and gonidia: on the dark-coloured 

 proto- or prothallus, he noted small nestling groups of green cells which 

 he, at that time, regarded as direct outgrowths from the lichen hyphae. 

 These gonidial cells, increasing by division, multiplied gradually and 

 gathered into a connected zone. He also observed that the hyphae in 

 contact with the gonidia became more thin-walled and produced many new 

 branches. Some of these newly formed branches grow upwards and form 

 the cortex, others grow downwards and build up the medulla or pith; the 

 filaments at the circumference continue to advance and may start new 

 centres of gonidial activity (Fig. 38 B). In many species,- however, this 

 prothallus or, as it is usually termed at this stage, the hypothallus, be- 

 comes very soon overgrown and obscured by the vigorous increase of the 

 first formed symbiotic tissue and can barely be seen as a white or dark line 

 bordering the thallus (Fig. 39). Schwendener^ has stated that probably 

 only lichens that develop from the spore are distinguished by a proto- 

 thallus, and that those arising from soredia do not form these first creeping 

 filaments. 



Fig. 38 A. Hypothallus of Rhhocarpon 

 confervoides DC, from the extreme edge, 

 with loose gonidia x 600. 



^ Zukal 1895. 



' Schwendener 1866. 



' Schwendener 1863. 



