KEV. J. M. CROMBIE ON THE ALGO-LrCHEtf IIYPOTIIESIS. 2G1 



conclusion that the lichen-gonidia are unicellular or filamentose 

 Algae, and the lichen-thallus a parasitic Fungus. The various 

 Algal types which he regards as constituting the gonidia he 

 includes under two groups, viz. the " PhycochromaceDS " and the 

 " Chlorophyllacese," so called from the colour of their respective 

 cell-contents. To the former group, that with bluish-green 

 cellules, he assigned five Algal types, viz. : — 1. Sirosiphoneae ; 2. 

 Rivulariae ; 3. Scytonemeae ; 4. JN'ostochaceae ; 5. Chroococcaceae : 

 and to the latter group, that with chlorophyll-green cellules, ho 

 assigns three Algal types, viz.: — 6. Confervaceae; 7. Chroolepideae ; 

 8. Palmellaceae. Moreover, failing in his researches to observe 

 any genetic connexion between the hyphae and the gonidia, and 

 unable otherwise to account for the presence of the gonidia in the 

 lichen-thallus, he propounded the theory now so well known as 

 the " Schwendenerian hypothesis." This, briefly stated in his 

 own words, is as follows : — " As the result of my researches," 

 he says, " all these (lichen) growths are not simple plants — not 

 individuals in the usual sense of the term ; they are rather colo- 

 nies, which consist of hundreds and thousands of individuals, of 

 which, however, only one acts as master, while the others, in per- 

 petual captivity, provide nourishment for themselves and their 

 master. This master is a fungus of the order Ascomycetes, a 

 parasite which is accustomed to live upon the work of others ; 

 its slaves are green algals, which it has sought out or indeed 

 caught hold of, and forced into its service. It surrounds them, 

 as a spider does its prey, with a fibrous net of narrow meshes, 

 which is gradually converted into an impenetrable covering. 

 While, however, the spider sucks its prey and leaves it lying dead, 

 the fungus incites the Algae taken in its web to more rapid 

 activity, nay, to more vigorous increase." In the conclusion of 

 the latter paper of Schwendener, above cited, he argues that the 

 algal nature of the lichen-gonidia, which he maintains has been 

 established in the cases he has reviewed, is extremely probable 

 also in every other case, and that consequently the gonidium is 

 not, as previously supposed, a self-developed organ of the lichen. 

 But though not thus to be regarded, gonidia, he says, would still, 

 in a physiological point of view, remain as instruments of assi- 

 milation and of asexual increase. For, although they have not 

 the power in themselves to form a thallus, they are an essential 

 constituent of it, and are undeniably the most important, though 

 not the only, ministers of nutrition to the composite plant called 



