THE FUNGI 249 



of the fruit is opened at the apex, and the edges gradually 

 pushed back as the thecium expands. 



The development of the apothecium may go on for 

 a very long time, even for years in some cases, new 

 asci arising towards the exterior margin. Each ascus at 

 a certain stage of development contains in its protoplasm 

 a single nucleus, which subsequently undergoes repeated 

 division, into two, four, and eight. When the full 

 number is attained, a cell is formed around each nucleus, 

 and these cells become the eight ascospores. The con- 

 tents of the ascus are not, however, completely used up 

 in the process of spore-formation ; a certain part remains 

 over, lying between the spores ; this unused substance 

 becomes gelatinous, and on taking up water tends to 

 swell, and so to burst the aseus. The dissemination of 

 the spores, however, is not entirely due to the pressure 

 from within the asci. The whole thecium, including 

 the paraphyses, endeavours to expand laterally when 

 wetted, and this expansion is resisted by the rim of tissue 

 at the edge. Hence the asci are subjected to very con- 

 siderable pressure, the result of which is that those 

 which are ripe dehisce at the top, expelling all the eight 

 spores with considerable force, so that they are shot up 

 as much as a centimetre into the air. The asci open 

 successively as they become mature, the dehiscence taking 

 place whenever wet weather occurs. 



The ascospores of Physcia, we have seen, can only 

 complete their germination under natural conditions, 

 and form a new Lichen-thallus, if they come into contact 

 with the cells of Cystococcus, with which they can enter 

 into partnership. The same applies to all Lichens, each 

 having its own particular Alga. 



There is another form of fructification consisting of 



