822 



fitted to stand a strong light, than an alga which is suddenly torn 

 from its habitat in deep water. 



But even if the alga is destroyed by unfavourable biologicai 

 conditions, the reproductive cells (tetraspores, carpospores etc.) which 

 it possibly carries may be able to survive the transport. 



If an alga has finally surmounted these various difficulties and 

 been washed ashore, it is still far from being settled among the 

 flora of the island. In order to become so, it must be able to fix 

 itself. A great many algæ are certainly able to do so; I may just 

 mention such species as Ceramium, Polysiphonia, Callithamnion, Ec- 

 tocarpns, Sphacelaria, Cladophora, Acrosiphonin etc. 



There is hardly any doubt, that all species belonging to these 

 very rich genera are normally supplied with rhizoids, or at any 

 rate are very apl to form them. If therefore one of these algæ when 

 it is washed ashore, sticks to something or other, for instance to an 

 alga growing there, it will perhaps only be a shorl time before it 

 forms rhizoids, which can fasten the plant to its subtratum. Even 

 very small fragments of several algæ seem to be able to produce 

 new individuals. Thus Oltmanns mentions (1. c), that he has cut 

 shoots of Polysiphonia and Ceramium into very small pieces which 

 quickly formed rhizoids and gradually developed into small piants. 

 In different papers Tobler^ also tells us that even one cell of some 

 Rhodomelaceæ can develop into new individuals. By cultivating a 

 Dasya elegans he observed, that the alga divided after a short time 

 into many small pieces, which gradually developed into small piants 

 in the culture-glass. He therefore supposes that the algæ must some- 

 times have a similar reproductive power in the sea. 



On the other band there certainly are many algæ which can- 

 not take root again , when they have once been torn from their 

 subtratum. That it may be possible for such algæ to immigrate 

 into another region, they must necessarily carry reproductive cells 

 within them. When an alga with more or less ripe tetraspores, 

 carpospores, etc. is torn from its habitat, nothing prevents us from 

 supposing, that after having floated for a shorter or longer time it 

 can succeed in carrying the spores to another habitat. In the new 

 region the spores can thrive, if the external conditions are suit- 

 able to the species. 



* Tobler. F. , Zerfall und Reproduktionsvermogen des Thailus einer Rhodo- 

 melacee. (Rerichte d. deut. bot. Ges. Rd. 20, 1902.) 



Ueber Eigenwachsthum der Zelle und Pflaiizeiiform. V^eisuche und 



Studien an Meeresalgen. (Pringsh. Jalirb. Rd. 39, 1904.) 



