CHAPTER XVI 



REPRODUCTION AMONG THE ALG.E 





JUST as the vegetative structure of the Algae shows progressive 

 stages in complexity, so also do the processes of reproduction, 

 although specialisation in the one respect does not always go 

 hand in hand with specialisation in the other. Thus Cladophora, 

 with its markedly differentiated vegetative system, shows the 

 same simple reproductive processes as does Ulothrix, and 

 Ectocarpus is scarcely more advanced. Our consideration of 

 the life-history of Chlamydomonas and its allies (Chapter XIV) 

 has already shown that new individuals may be formed in various 

 ways, and, if we survey the Algae as a whole, it is especially in 

 relation to the sexual method of reproduction that progressive 

 differentiation becomes most apparent. It will be convenient 

 at the outset to study an Alga like Ulothrix, which, while exempli- 

 fying the various types of reproductive processes customary 

 in the group, shows them in their simplest form. 



At times of active growth this Alga reproduces vegetatively 

 by the mere splitting or fragmentation of its filaments into short 

 lengths which develop into new threads. Not infrequently, 

 however, a more specialised mode of vegetative multiplication 

 obtains, spoken of as asexual reproduction owing to its general 

 resemblance to the sexual method except for the absence of 

 fusion. Both the sexual and asexual reproductive cells are 

 motile, although the ordinary Ulothrix-filament is without 

 any power of movement. 



Asexual reproduction may take place in some or all of the 

 cells of a filament. At its commencement, the protoplasts rounc 

 off slightly and thereupon usually divide, along successive 

 planes at right-angles to one another, into 2, 4, or even 8 separate 

 parts, the number depending upon the size of the cell (Fig. 115, b). 



212 



