120 PRACTICAL BOTANY. 



margin of the apothecium (the excipulum), consisting of less 

 swelling tissue, resists the increase of bulk of the hymenium, and 

 pressure is thus established. 



Note that the ends of the paraphyses and of the older asci have 

 a yellowish-brown colour, similar to that of the surface of the 

 rest of the thallus : treat sections with potash solution and warm : 

 the parts coloured yellow assume a pink colour, which diffuses 

 out into the solution, and also into other tissues of the thallus : 

 neutralise with acetic acid, the colour will disappear, but may be 

 again produced by adding potash. Compare with this the well- 

 known properties of litmus, which is produced from Roccella 

 tinctoria and other Lichens. 



V. Transverse sections may also be cut through 

 the hymenium : treat as before, and note the asci, 

 which appear of circular outline, of relatively large 

 size, and contain the highly refractive spores : the 

 paraphyses, of relatively small bulk, closely packed 

 round them. 



VI. Observe the spores in detail : this may be done 

 by inverting the surface of an apothecium (previously 

 kept dry) in a drop of water on a slide, when spores 

 will be ejected, by means of the pressure due to 

 swelling as above noted: mount these in water, and 

 examine them under a high power : observe their oval 

 form, and the presence of a highly refractive body at 

 either end, the two being connected by a fine strand : 

 stain with an iodine solution, the highly refractive 

 bodies as well as the connecting strand will stain 

 yellowish-brown (protoplasmic body), the rest of the 

 spore is not distinctly stained (cell-wall). 



There is considerable variety in the number of the spores 

 produced in each ascus in various Lichens, and they attain in some 

 cases great complexity of structure, as well as considerable size. 



Compare Megalospora, where in each ascus only one large 



