564 



HUTCHINSON'S POPULAR BOTANY 



FIG. 712. BLADDER WRACK. 



Oosphere being fertilized by 

 antli 



One is loy the fusion of two planogametes, as in 

 Protococcoidese ; the other is by the enlargement 

 of one cell into an oogonium containing a single 

 cell, which is fertilized by a spermatozoid formed 

 in another cell of the same or another filament. 

 The Class CONJUGATE consists of fresh-water 

 Algse very similar in appearance to the single- 

 celled and filamentous forms of CHLOROPHYCE.E, 

 but reproduction is effected asexually by simple 

 cell-division, and sexually by the conjugation of 

 two apparently similar cells resulting in the forma- 

 tion of a zygosperm. 



The most familiar representatives of the class 

 are the Yoke-threads (Zygnema, Spirogyra) and 



the Desmids, the former consisting of long green hair-like filaments 

 growing in fresh-water, and the Desmids single-celled microscopic 

 water-plants. Zygnema consists of a single row of transparent cylindri- 

 cal cells, joined end to end, and ornamented within by spiral bands of 

 endochrome which gives it a beautiful appearance under the microscope. 

 They float loosely in or on the water and have no attachment suckers. 

 Single cells broken off the thread have the power by cell-division to grow 

 into long filaments, that is, complete plants. When two filaments come 

 together so that their cell-walls are nearly in contact, conjugation takes 

 place. A protuberance grows out from each of the opposite cells, and these 

 shoots meet halfway between the threads, the cell-walls of the tips open to 

 form a connecting tube between the two plants, and the contents of one cell 

 pass into its neighbour, where they coalesce with the contents of that cell. 

 There is complete union of this mass of protoplasm, which then rounds off 



into a sphere or an ellipse. It has become 

 a zygosperm. In some species the union 

 of protoplasm takes place in the connect- 

 ing tube; in others no tube is formed, 



^^ but the two plants come in contact by a 



( -^ n ^In/) * knee-like bend in each which brings two 



fAJ^Kr^' '' ce ^ s i R touch with each other, then the 



j^^f J* cell-walls between them disappear and a 



zygosperm is formed in one cell by the 

 union of the contents of both. Occasion- 

 ally two cells on the same plant will form 

 a zygosperm by each sending out a tube 

 which meets with the other and effects a 

 combination of the two protoplasm masses. 

 However formed, these zygosperm s ger- 

 minate and give rise directly to a fila- 



Fio. 713. BLADDER WRACK. 



Oogonium distended by its eight oospheres, and 

 split and release them. 



