THE ALGAE 



89 



Zygospore 



Unconjugated 



cell 



Conjugation 

 canal 



Meanwhile the contents of the conjugating cell which is about to function 

 as the male gamete contract away from the outer cell wall but without losing 

 contact with the female. In this process many contractile vacuoles are formed, 

 which pump water from the central cell vacuole into the space between the 

 cell wall and the contracting cytoplasm. Subsequently the female gamete 

 also contracts from the conjugation tube, leaving a space into which the male 

 gamete passes, enters the other cell and fuses with the female gamete. It 

 appears that the migration of the male gamete along the conjugation tube 

 and the contraction of the female 

 are brought about by the pump- 

 ing action of contractile vacu- 

 oles. In Moiigeotia and certain 

 allied genera the process is 

 slightly different, both gametes 

 passing into the conjugation 

 tube, where the zygote is 

 formed. Such types are there- 

 fore isogamous. 



The active gamete is looked 

 upon as the male and the pas- 

 sive one as the female, although 

 they do not show any morpho- 

 logical differences. We have 

 therefore here a case of physi- 

 ological heterogamy. In some 

 species of Spirogyra all the male 

 gametes are formed in the cells 

 of one filament, and all the 

 females in the other, with the 

 result that after conjugation we 

 find one filament entirely devoid 

 of cell contents, while the other 



contains all the zygotes. In other species some cells function as males and 

 others as females, with the result that the zygotes are distributed between 

 both filaments. The former species are dioecious and the latter are 

 monoecious. 



There is also another type of conjugation called the chain type, in 

 which adjacent cells of the same filament unite through tubes which are formed 

 like side-links between cell and cell. In this case alternate cells behave as 

 males and females respectively, and there is no sexual differentiation between 

 the filaments as a whole. In some species this method is the rule. 



Whatever the position of the formation of the zygote its subsequent 

 development is the same. Immediately after the fusion of the gamete nuclei 

 a zygospore is formed. A zygospore is a zygote enclosed in a thick wall. 

 It is specially adapted for withstanding dry conditions, and is easily recognized 

 by the thick yellow-brown wall which is usually produced. In Spirogyra 



4A 



Fig 



-Spirogyra sp. Sexual reproduction, ladder 

 Two completed zygospores appear in the 

 left-hand filament. In the right-hand filament 

 one cell has remained unpaired. 



69. 

 type. 



