

LICHENS. 121 



unicellular spore is formed ; Pertusaria, with two, four, or six in 

 different species ; Peltigera, Graphis, and Collema, in which ea9h 

 spore is composed of four or more cells (sporae compositae). 



VII. The discovery of the sexuality in certain of 

 the Lichens is one of the most marked observations of 

 recent years, and attempts should be made to see at 

 least the more important organs concerned in the process, 

 which are on the one hand the spermogonia (male), 

 on the other the ascogones (female) : these may both be 

 seen in species of Collema collected in early spring ; while 

 the spermogonia, which in other Lichens are often 

 difficult to find, may be observed in any specimen of 

 " Iceland Moss " (Cetraria Islandica) as sold in druggists' 

 shops. 



Observe on specimens of Cetraria a fine fringe of 

 minute teeth along the margin, each of these contains 

 a spermogonium at its apex : embed a portion of a 

 thallus bearing these teeth, and cut fine transverse 

 sections of it : the teeth will thus be cut longitudinally. 

 Mount in glycerine, having previously allowed the 

 sections to swell in water, and look over the sections 

 under a low power to find examples of spermogonia cut 

 longitudinally, which will appear as flask-like cavities. 

 Having found one of these, examine it under a high 

 power, and observe 



1. The general structure of the thallus corre- 

 sponding to the heteromerous type above described 

 (Parmelia). 



2. The hollow spermogonium, composed exclusively 

 of the Fungal constituent of the Lichen, the hyphse being 

 closely packed and pointing radially inwards to the 

 cavity, and giving off at their apices by abstriction the 



