274 KEY. J. M. CBOMBIE OK THE ALGO-BTCHEtf HYPOTHESIS. 



speaking of the gonidimia of Endocarpon miniatum, he says : — 

 " In these cells (*. e. of the cortical stratum) is produced the 

 chlorophyll." Similarly in Ny lander's ' Synopsis Methodica 

 Lichenum' (1858), tab. i. if. 1, 3, 4, 6, the sections of the dif- 

 ferent thalli there given, belonging to several tribes and genera, 

 show, even at first sight, the gonidia enclosed in the thalline 

 cellules under conditions in which they could not have entered 

 from without, but must have originated in the cellules themselves. 

 It is rather singular that both in the case of Tulasne and of 

 Nylander these statements and figures should have until recently 

 been entirely overlooked, and their import not comprehended 

 either by the supporters or the opponents of the Algo-lichen 

 theory. In them, however, we obtain the clue which is to guide 

 us in our further inquiries upon the subject. 



As already shown, lichens can scarcely in ordinary circum- 

 stances be successfully cultivated from the spores beyond a certain 

 stage (hypothalline) of growth. This, however, is the less to be 

 regretted, because in nature itself (and nature is always its own 

 best interpreter) we can in some instances behold the whole 

 process of the evolution of the thallus. More especially is this 

 the case with respect to crustaceous species growing pure upon 

 such substrata as quartz rock, flint stones, and young bark of 

 trees *. In the various Lecanoras and Lecideas which occur on 

 the surfaces of these, we can readily trace the life-history of a 

 lichen, in so far as the vegetative organs are concerned, through 

 every stage from the germinating spore to the perfect plant, 

 in the manner now to be described. On germination, as may 

 easily be seen in spore-culture (and indeed not unfrequently 

 even in the apothecia themselves), the spore sends forth from 

 the endospore, * in various ways t, a germinating filament or 

 filaments called the prothallus. This speedily passes into 



* Where the substratum is not pure but shows Tarious heterogeneous growths, 

 as is usually the case on the trunks of old trees, we often find, especially in the 

 case of Opegraphas, Arthonias, and Verrucarias, different forms of gonidia 

 immixed and confused, apparently constituting the same thallus. Hence 

 Bornet, as already noticed, and more recently Almquist (in ' Monographia 

 Arthoniarum Scandinavia;,' 1880), erroneously assume the presence of diverse 

 gonidia in the same lichen — a polymorphism which is entirely owing to " the 

 struggle for existence " amongst different species. 



t Numerous illustrations of germinating spores belonging to various tribes 

 and genera of Lichens may be seen in the Tables appended to the ' Memoire ' of 

 Tulasne already cited. 



