REV. J. M. CROMBIE ON THE ALGtO-LICHEV HYPOTHESIS. 277 



quite ignored or not rightly discriminated by Schwendenerians, 

 who speak as if only two elements entered into the structure of 

 the lichen, viz. hyphse and gonidia, whereas to these must unques- 

 tionably be added a third element, viz. the cellular cortical tissue *. 

 But not only in young crustaceous thalli do we thus perceive 

 the gonidia originating in the cellules of the first-formed paren- 

 chymatous tissue, but even in adult thalli amongst the higher 

 foliaceous lichens we can sometimes observe growing gonidia in 

 the cortical cellules. This is the case in such species as Physcia 

 pulverulenta, Ph. litlwtea, TJmbilicaria pustulata, Psoroma hyp- 

 norum, &c, in which the thalli have the cellules very distinct and 

 evident (not confused or obliterated), and are consequently best 

 adapted fur examination, So also the intracellular origin of the 

 gonidia is equally apparent in the thalliue margin of lecanorine 

 apothecia, and even of biatorine apothecia which have gonidia 

 intruded into their proper margin, where they are seen in a young 

 or growing state towards its external and youngest portion. As 

 to the origin of the gonidia in the gonidial stratum, this also 

 admits of a ready and simple interpretation. " The cortical stra- 

 tum," as observed by Ny lander in ' Flora,' 1877, p. 354, "gradu- 

 ally increasing and expanding, is at the same time in like propor- 

 tion dissolved (or resorbed, as it is termed in physiology) beneath, 

 and the gonidia consequently become free." In this condition, 

 whether enclosed in cellules or variously conjoined or discrete, 

 they constitute the narrow gonidial stratum, which is situated 

 between the lower portion of the cortical and the upper portion 

 of the medullary strata. This latter stratum is produced from 

 the cortical cellular tissue and arises in the thallus either before 

 or at the same time as the gonidial stratum, so that though 

 situated beneath this, it is not posterior in its formation. When 

 the medulla is filamentose (for it is frequently crustaceous, as it 

 subsequently becomes in the specimens exhibited, and is also 



* This origin of the gonidia, which was first, as we hare already stated, 

 observed by Tulasne, was further confirmed by Arcangeli (" Sulla questione del 

 gonidi ") in Nuov. Giorn. Bot. Ital 1875, where, after recording various obser- 

 vations made by him upon several lichens, he says that " the gonidia arise from 

 organs which have a more or less pseudo-parenchymatous structure." It is. 

 however, to Dr. Nylander, whose long services to lichenological science are so 

 valuable and extensive, that we owe the actual demonstration of the origin of 

 the gonidia and of their relation to the other elements of the thallus (vide 

 'Flora' from 1877, paxsi?»i). 



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