LICHEN ASCI AND SPORES 189 



The development of the thickened wall of poiarilocular spores has been 

 studied by Hue', who contends however that there is no true septation in 

 the colourless spores so long as the central canal remains open. According 

 to his observations the wall of the young spore is formed of a thin tegument, 

 everywhere equal in thickness, and consisting of concentric layers. This 

 tegument becomes continually thicker at the equator of the spore by the 

 addition of new layers from the interior, and the protoplasmic contents are 

 compressed into a gradually diminishing space. In the end the wall almost 

 touches at the centre, and the spore consists of two polar cell-cavities with 

 a narrow open passage between. A median line pierced by the canal is 

 frequently seen. In a few species there is a second constriction cleavage 

 and the spore becomes quadrilocular. 



Hue insists that this spore should be regarded as only one- celled; for 

 though the walls may touch at the centre, he says they never coalesce. He 

 has unfortunately given no cytological observations as to whether the spore 

 is uni- or binucleate. 



In Xanthoria parietina, one of the species with characteristic polari- 

 bilocular spores, germination, it would seem, takes place mostly at one end 

 only of the spore, though a germinating tube issues at both ends frequently 

 enough to suggest that the spore is binucleate and two-celled. The absence 

 of germination from one or other of the cells only may probably be due to 

 the drain on their small resources. Hue has cited the rarity of such instances 

 of double germination in support of his view of the one-celled nature of the 

 spore. He instances that out of fifteen spores, Tulasne- has figured only 

 three that have germinated at each end; Bornet* figures one in seven with 

 the double germination and Bonnier* one in sixteen spores. 



Further evidence is wanted as to the nuclear history of these hyaline 

 spores. In the case of the brown spores, which show the same thickening 

 of the wall and restricted cell-cavity, though with a distinct median septum, 

 nuclear division was observed by Rene Maire' before septation in one such 

 species, Anaptychia ciliaris. 



II. SECONDARY SPORES 



A. Reproduction by Oidia 



In certain conditions of nutrition, fungal hyphae break up into separate 

 cells, each of which functions as a reproductive conidium or oidium, which 

 on germination forms new hyphae. Neubner" has demonstrated a similar 

 process in the hyphae of the Caliciaceae and compares it with the oidial 

 formation described by Brefeld' in the Basidiomycetes. 



' Hue igii^. 2 -pulasne 1852. 3 Bornet 1873. •• Bonnier 1889^. 



° Maire 1905. * Neubner 1893. ' Brefeld 1889. 



