FRESHWATER ALGM. 145 



FEESHWATEE ALG^.* 



BY A. W. WILLS, F.C.S. 



(Continued from page 117.) 



We must now turn to the very important question of the modes of 

 multiphcation and reproduction of these singular plants. 



In the simplest forms of unicellular Algae, (Palmellacece and Volvoc- 

 inete,) the only mode by which most of the genera seem to be perpetuated 

 consists of the conversion or breaking up of the contents of certain cells 

 into small oval bodies, (zoospores,) which move freely through the water 

 for a while by means of dehcate lashes or cilia, and then, after losing 

 these motile organs, undergo cell-division into fresh individuals, identical 

 with the parents. [Plate I., Fig. 2.] 



In Df«Ht(f?!'«ce«, to this process is superadded that of reproduction by 

 the formation of large " resting-spores " resulting from the commingling 

 of the contents of two separate individuals in the space between their 

 adjacent sides. [Plate II., Fig. 7.] Such spores are capable of retaining 

 their vitality for a long period, and then producing fi'om their contents 

 a fresh generation of plants of their specific kind ; while, on the other 

 hand, zoospores undoubtedly perish, unless placed in conditions 

 favourable to their immediate germination. 



In many of the higher order of Algas (freshwater and marine) both 

 these phenomena can be traced, and it appears probable that in all cases 

 the development of spores alone constitutes a true sexual reproduction 

 of the species, comparable to that which prevails among Phanerogams, 

 whereby its indefinite perpetuation is secured ; while the so-called 

 " reproduction " by zoospores is rather to be compared to the increase of 

 flowering plants by buds or offsets, or artificially by cuttings, a process 

 by which, as is behoved by most florists, while the inultiplication of 

 individuals is accomphshed, the continuation of the species is not 

 secured beyond certain limits. 



It is probable that even in those genera in which no such sexuality 

 or polarisation of cell-contents has been observed, it has yet to be 

 discovered, much as the fine researches of Messrs. Dalhnger and Drysdale 

 "On the Life History of Monads" have proved, that after the succession 

 of many generations of asexual individuals among the lowest Infusoria, a 

 sexual process suddenly intervenes. 



The production of spores in Besmidiacea is a most interesting 

 phenomenon, and will be best understood by those to whom it is 



* Description of Figures in Plate II. 

 Fig. 7.— Formation of restiug-spore of Micrasterias rotata, from contents of two 



plants. 

 Fig. 8. — Vaucheria ornithocephala. 

 Fig. 9.— Vaucheria racemosa, showing " horns " and flask-shaped cells, the latter 



containing spores. 

 Fig. 10.— Formation of zoospores fro'n contents of single cells in aSdogoniuni. 

 Fig. 11.— Formation of spores in CEdogonium, the contents of the enlarged cell 



being fertilised by spcimatozoids developed in dwarf parasitic male, or anther- 



idial plants. 



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