184 REPRODUCTION 



The " proper margin " reaches its highest development in the lecideine 

 and graphideine types. It is less prominent or often almost entirely replaced 

 when the thalline margin is superadded, except in genera such as Thelotrema 

 and Diploschistes which have distinct " double margins." 



There is an unusual type of apothecium in the genus Gyrophora. The 

 fruit is lecideine, the thalline gonidia taking no part in the development. 



The growth of the initial ascogenous tissue, 

 according to Lindau 1 , is constantly towards 

 the periphery of the disc so that a weak 

 spot arises in the centre which is promptly 

 filled by a vigorous sterile growth of para- 

 ^*~** s ? c physes. This process is repeated from new 



I<ig. 103. Apothecial gyrose discs of * 



Gyrophora cylindrica Ach. x 12 (after centres again and again, resulting in the 



irregularly concentric lines of sterile and 



fertile areas of the "gyrose" fruit (Fig. 103). The paraphyses soon become 

 black at the tips. Asci are not formed until the ascogenous layer has ac- 

 quired a certain degree of stability, and spores are accordingly present only 

 in advanced stages of growth. 



G. LICHEN ASCI AND SPORES 



a. HISTORICAL. The presence of spores, as such, in the lichen fruit was 

 first established by Hedwig 2 in Anaptychia (Physcia) ciliaris. He rightly 

 judged the minute bodies to be the "semina" of the plant. In that species 

 they are fairly large, measuring about 50 /* long and 24/1 thick, and as they 

 are very dark in colour when mature, they stand out conspicuously from the 

 surrounding colourless tissue of the hymenium. Acharius 3 also took note of 

 these " semina " and happily replaced the term by that of " spores." They 

 may be produced, he states, in a compact nucleus (Sphaeropkoron), in a naked 

 disc (Caliciuni), or they may be embedded in the disc (Opegrapha and Lecidea). 

 Sprengel 4 opined that the spores which he figures were true seeds, though 

 he allows that there had been no record of their development into new plants. 

 Luyken 8 made a further contribution to the subject by dividing lichens into 

 gymnocarpous and angiocarpous forms, according as the spores, enclosed 

 in cells or vesicles (thecae), were borne in an open disc or in a closed peri- 

 thecium. 



In his Systema of lichen genera Eschweiler 6 , some years later, described 

 and figured the spores as " thecae " enclosed in cylindrical asci. F6e r in 

 contemporary works gave special prominence to the colour and form of the 

 spores in all the lichens dealt with. 



1 Lindau 1899. a Hedwig 1784. 3 Acharius 1803. * Sprengel 1807. 



8 Luyken 1809. 6 Eschweiler 1824. 7 Fee 1824. 



