

LICHENS. 119 



IV. Having carefully noted the form of the apothe- 

 cium of Parmelia, and its attachment to the thallus, 

 cut thin vertical sections through it, and, mounting in 

 glycerine without previously allowing them to swell 

 in water, observe, under a low power 



1. That the structure of the part of the thallus 

 which bears the apothecia is similar to that described 

 above. 



2. That the lower stratum of the tissue composing 

 the cup is similar to the above, but note especially the 

 very considerable masses of gonidia immediately below 

 the upper stratum of the apothecium. 



3. That the upper stratum of the apothecium consists 

 of closely packed, more or less club-shaped elements, 

 regularly arranged perpendicularly to the surface : this 

 is the hymenium, of which two constituents are to be 

 distinguished 



a. The asci, which may be recognised as relatively 

 wide, club-shaped cells ; the contents vary according to 

 the stage of development : when mature each contains 

 eight oval spores. Note that various stages of deve- 

 lopment are to be seen in the same apothecium. 



b. The paraphyses, which are relatively narrow, and 

 take no direct part in reproduction. 



Treat a fresh section through an apothecium with 

 iodine solution, and note that the cell-walls of the 

 hymenium assume a blue coloration. 



Treat some sections through an apothecium with water, and 

 observe how greatly the hymenium swells, so that it is thrown 

 into numerous folds : it may be remembered that this property of 

 swelling of the constituents of the hymenium is connected with 

 the rupture, and extrusion of the spores when mature ; since the 



