150 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



fundamentally a similar relationship to that which we have seen in Lemanea and 

 Batrachospermum. 



Polysiphonia Binderi l exhibits peculiar features. The development of the plant 

 begins with a cylindrical germ-shoot which altogether resembles that of the type of the 

 genus in structure and configuration. Upon this one or many flat structures arise 

 laterally, and these lay themselves like a crust upon the surface of other Algae, chiefly 

 species of Codium, and may be regarded as composed of a number of threads of Poly- 

 siphonia united together in one plane. When the formation of propagative organs has 

 to take place free threads of the Polysiphonia are again produced and they bear tetra- 

 spores. This case is specially interesting. It is clear that the crust-form of the thallus 

 is a secondary adaptation which brings about the firm anchoring of the organ to the 

 substratum ; the adaptation does not appear however in the first stages of germination. 

 Had the crust appeared in the germination of the spore, which it is quite possible to 

 conceive, we should have had a case somewhat like that of Dumontia, but in that 

 genus the structure of the disc points to the retention of a more primitive character. 



Other Algae behave in a manner similar to those already described, but I must 

 content myself here with a brief reference to them. The pro-embryo of Chara is 

 described in every detail in the larger text-books 2 . In the Sphacelarieae ' an 

 anchoring disc is usually formed in germination upon which the cylindrical shoots 

 which do the work of assimilation and bear the fructification are then produced. 

 The anchoring disc is evidently made up of creeping cell-threads which are fused 

 with one another as they are in Polysiphonia Binderi, and have therefore experienced 

 further differentiation about which however I cannot pause to speak. Battersia 

 mirabilis is specially interesting because the anchoring disc, which I regard as an 

 adaptation arising secondarily 4 , appears as the real vegetative body of the plant, and 

 upon it the shoots bearing the fructification arise as short appendages in the same 

 way as they do in Ephemerum, Lejeunia Metzgeriopsis, and others amongst the 

 Bryophyta. 



We see then in all these Algae that the juvenile stages exhibit two 

 peculiarities either separated or together : on the one hand a primitive 

 configuration which conforms with that of allied forms (Polysiphonia 

 Binderi) or presumptive ancestors (Bratachospermum) ; on the other 

 hand adaptations to which we cannot attribute a phylogenetic significance 

 and which in these Thallophyta chiefly, although not always, stand in 

 connexion with their fixation to the substratum. 



' This plant has been reckoned as the type of a special genus Placophora, because of its vegeta- 

 tive characters of adaptation, but this is opposed to all right principles of classification. See Flora, 

 1889, p. 3, where the literature is cited. Figures of this plant will be found in my ' Pflanzen- 

 biologische Schilderungen,' i. p. 64, figs. 69, 70, 71. 



2 Goebel, Outlines of Classification, p. 53. 



3 See Reinke, Ubersicht der bisher bekannten Sphacelariaceen, in Ber. der deutsch. bot. Ges. viii. 

 p. 201 ; also Beitrage zur vergl. Anatomie nnd Morphologic der Sphacelariaceen, in Bibliotheca 

 botanica, Heft 23, where the literature is quoted. 



4 Amongst Ectocarpeae a similar formation of anchoring discs occurs, but it is not general. 

 See my ' Pflanzenbiologische Schilderungen,' i. p. 163. 



