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A TEXTBOOK OF THEORETICAL BOTANY 



We see, therefore, that in Ulothrix we have an example of perfect isogamy , 

 in which the two gametes are of identical size and structure, though from the 

 fact that gametes from the same mother cell will not fuse together we realize 



that there must be some 

 physiological diflFerence 

 between them, which may 

 explain why gametes of 

 different origin are at- 

 tracted to one another. 



Asexual reproduction 

 in Ulothrix takes place 

 chiefly in the winter, and 

 it is at this time of the 

 year that the growth of 

 this Alga is most active. 

 During the summer re- 

 production is by sexual 

 means, and it is interesting 

 to note that after the forma- 

 tion of the zygospore ger- 

 mination may proceed very 

 slowly, and it may rest for 

 a considerable period whilst 

 still unicellular. It is 

 only when conditions be- 

 come more favourable 

 that the new filament is 

 produced. We see, there- 

 fore, that here, as in 

 many other lower organ- 

 isms, sexual reproduction 

 unfavourable to vegetative 



Fig. 



63. — Ulothrix zofiato. A, Release of zoospores from 

 the cells of the filament. B, Macrozoospore. C, 

 Microzoospore. D, Release of gametes. E, Gamete. 

 F and G, Conjugation stages. H, Motile zygote. 

 J, Zygospore. 



may be 

 growth. 



associated with conditions which are 



Oedogoniales 



The Oedogoniales are Chlorophyceae in which both the zoospores and the 

 antherozoids are characterized by having a whorl of many flagella at their 

 anterior end. Sexual reproduction is always of an advanced type ; the male 

 organs, or antheridia, giving rise to antherozoids, and the female organs, or 

 oogonia, each producing a single oosphere. 



The vegetative cells are elongated and form a filament ; growth is by 

 division of the cells, and in this process there is a characteristic annular 

 splitting of the cell wall prior to the formation of the new cell. The filaments 

 may be simple or branched. 



We shall consider as our type the common Alga Oedogonium. 



