50 CONSTITUENTS OF THE LICHEN THALLUS 



Pertusaria communis possesses a very large simple spore, but it is multi- 

 nucleate and germinates with about 100 tubes which reach their ultimate 

 width of 3 to 4 /i before they emerge from the exospore. The hyphae 

 encircle the spore, and an opaque thalline growth is quickly formed from 

 which rise terminal hyphal branches. In ten weeks the differentiation into 

 medulla and cortex was reached, and in five months the hyphal thallus 

 measured 4 mm. in diameter and I to 2 mm. in thickness. 



Moller instituted a comparison between the thalli he obtained from the 

 spores and those from the spermatia of another crustaceous lichen, Buellia 

 punctiformis (B. myriocarpd). After germination had taken place the hyphae 

 from the spermatia grew at first more quickly than those from the ascospores, 

 but as soon as thallus formation began the latter caught up and, in eight 

 weeks, both thalli were of equal size. 



Another comparative culture with the spermatia and ascospores of 

 Opegrapha subsiderella gave similar results: the spores of that species are 

 elongate-fusiform and 6- to 8-septate; germination took place from the end 

 cells in two to three days after sowing. The germinating hyphae corre- 

 sponded exactly with those from the spermatia and growth was equally slow 

 in both. The middle cells of the spores may also produce germinating tubes, 

 but never more than about five were observed from any one spore. A 

 browning of the cortical layer was especially apparent in the hyphal culture 

 from another lichen, Graphis scripta : a clear brown colour gradually changing 

 to black appeared about the same period in all the cultures. 



The hyphae from the spores of Arthonia developed quickest of all: the 

 hyphae were very slender, but in three to four months the growth had reached 

 a diameter of 8 mm. In this plant there was the usual outgrowth of delicate 

 hyphae from the surface; no definite cortical layer appeared, but only a very 

 narrow line of more closely interwoven somewhat darker hyphae. Frequently, 

 from the surface of the original thallus, excrescences arose which were the 

 beginnings of further thalli. 



Tobler 1 experimenting with Xanthoria parietina gained very similar 

 results. The spores were grown in malt extract for ten days, then transferred 

 to gelatine. In three to five weeks there was formed an orbicular mycelial 

 felt about 3 mm. in diameter and 2 mm. thick. The mycelium was frequently 

 brownish even in healthy cultures, but the aerial hyphae which, at first, rose 

 above the surface were always colourless. After these latter disappeared a 

 distinct brownish tinge of the thallus was visible. In seven months it had 

 increased in size to 15 mm. in length, 7 mm. in width and 3 mm. thick with 

 a differentiation into three layers: a lower rather dense tissue representing 

 the pith, above that a layer of loose hyphae where the gonidial zone would 



1 Tobler 1909. 



