158 



REPRODUCTION 



frequently septate, especially towards the apex, and mostly slender, varying in 

 width from 1-4/"-, though Hue describes paraphyses in Aspicilia atroviolacea 

 as 8-I2/X thick. They may be thread-like throughout their length, or they 

 may widen towards the tips which are not infrequently coloured. Small 

 apical cells are often abstricted and lie loose on the epithecium, giving at 

 times a pruinose or powdered character to the disc. In some genera there 

 is a profuse branching of the paraphyses to form a dense protective epithecium 

 over the surface of the hymenium as in the genus Arthonia. 



The apothecia may be sessile and closely adnate to or even sunk in the 

 thallus, or they may be shortly stalked. The thalline margin shares generally 

 the characters of the thallus ; the disc is mostly of a firm consistency and is 

 light or dark in colour according to genus or species; most frequently it is 

 some shade of brown. Marginate apothecia, i.e. those with a thalline margin, 

 are often referred to as "lecanorine," that being a distinctive feature of 

 the genus Lecanora. In the immarginate series, with a proper margin 

 only, the texture may be soft and waxy, termed "biatorine" as in Biatora; 

 or hard and carbonaceous as in the genus Lecidea, and is then described as 

 "lecideine." 



In the subseries Graphidineae, the apothecium has the form of a very 



flat, roundish or irregular body entirely without 

 a margin, called an "ardella" as in Arthonia; 

 or more .generally it is an elongate narrow 

 " lirella," in which the disc is a mere slit 

 between two dark-coloured proper margins. 

 The hypothecium of the lirellae is sometimes 

 much reduced'and in that case the hymenium 

 rests directly on a thin layer above the thalline 

 tissue as in Graphis elegans (Fig. 89). 



Lichen fruits require abundant light, and 

 plants growing in the shade are mostly sterile. 

 Naturally, therefore, the reproductive bodies 

 are to be found on the best illuminated parts 

 of the thallus. In crustaceous and in most 

 foliose forms, they are variously situated on 

 the upper surface, wherever the light falls most directly. In the genera 

 Nephromium and Nephromopsis, on the contrary, they arise on the under sur- 

 face, though at the extreme margin, but as the fertile lobes eventually turn 

 upwards the apothecia as they mature become fully exposed. In shrubby 

 or fruticose lichens their position is lateral on the fronds, or more frequently 

 at or near the tips. 



b. Perithecia. The small closed perithecium is characteristic of the 

 Pyrenocarpeae which correspond with the Pyrenomycetes among fungi. It 



B 



Fig. 89. Graphis elegans Ach. A, 

 thallus and lirellae ; B, vertical 

 section of furrowed lirella. x ca. 

 5°- 



