Mr. G. H. K. Thwaites on Lichens. 387 



assent to the interpretation of the phenomena which he and 

 some other distinguished lichenologists give to them. 



It is well known that the ordinary lichen, as generally 

 recognized, discharges, when mature, its fruit (sporidia) from 

 portions of its upper surface, and that at the same time gonidial 

 geramge escape spontaneously from other parts of its structure. 

 These two productions (sporidia and gonidia) naturally be- 

 come scattered about in abundance and in close proximity 

 with one another ; thence it follows that they must come into 

 mutual contact not unfrequently, and more certainly so when 

 each of the kinds begins to extend itself by growth. Both 

 sporidia and gonidia are capable of developing for a certain 

 time, each independently and unconnected with the other; 

 and as they retain each its own character of tissue, so diffe- 

 rent in appearance from that of the other, a casual observer, 

 seeing them not in connexion, may easily arrive at the belief 

 that they are essentially distinct and by no means specifically 

 identical. To him there is therefore a jprimd facie appear- 

 ance of parasitic action when he sees the somewhat fungus-like 

 little detached germinating sporidia, when coming into contact 

 with portions of the alga-like gonidia, adhering organically to 

 them, continuino- to grow in such connexion, and for a certain 

 time in a seemingly irregular manner. It will be noticed, 

 however, by ihe candid investigator, that the subsequent more 

 mature development of the combined structures exhibits a very 

 different character. It will be seen, too, that it is essentially 

 different from what is observed in instances of real parasitism, 

 as generally understood ; for in these latter the parasite is 

 observed to be nourished at the exjjense of the organism upon 

 which it is growing, to be injurious to its health, and to 

 impair its beauty and symmetry. But how different, in con- 

 tradistinction to this, is the result brought about by the 

 so-called lichen-parasitism ! In this latter we observe that the 

 intimate coalescence of the two above-described organically 

 adhering kinds of tissue (sporidial and gonidial) eventuates 

 in a most beautiful symmetrical structure — the perfect lichen, 

 with its every part manifesting the possession of full health 

 and vigour. 



In the presence of these last-mentioned circumstances it 

 really seems to me that those who believe that the two kinds 

 of tissue above referred to belong respectively to two totally 

 distinct species of plants should, consistently with such belief, 

 acknowledge that the phenomenon ought to be looked upon as 

 an example of commensal ism rather than one of parasitism. 

 And, granting for a moment this to be the correct view, it 

 might surely be reasonably expected that, instead of finding, 



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