250 STRUCTURAL BOTANY 



between the spores ; this unused substance becomes 

 gelatinous, and on taking up water tends to swell, and 

 so to burst the ascus. The dissemination of the spores, 

 however, is not entirely due to the pressure from within 

 the asci. The whole hymenium, 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 considerable pres- 

 sure, 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 

 small flask-shaped receptacles called spermogonia. From 

 the walls of these organs filaments project into the 

 cavity, and from the ends of these filaments excessively 

 minute cells (the spermatia) are cut off. The whole 

 structure is not unlike the spermogonium of ^cidium 

 figured below (see Fig. 103). There is no certainty 

 as to the nature of these spermatia. They have been 

 regarded as male cells, and in a few Lichens evidence has 

 been adduced, to show that, by their means, the hypha from 

 which the asci are produced is fertilised. In a much 

 larger number of cases, however, the so-called spermatia 

 have been observed to germinate, and even to produce, 



