CONJUGA T^E 



265 



'endophytic in species of Spirogyra and Zygnema, have been mistaken 

 for zoospores. 



The usual mode of conjugation is the scalariform, between the cells 

 of two filaments lying side by side. The first stage is the putting out of 

 lateral protuberances at right angles to the axis of growth, from the cells 

 in each filament towards the corresponding cells in the other filament. 

 The protuberances put out by opposite cells at length meet ; the proto- 

 plasm-mass of each of the two cells has by this time begun to contract, 

 withdrawing itself from the cell-wall, and rounding itself off into an ellip- 

 soidal form, with expulsion of some 

 of its cell-sap, the chlorophyll- 

 bodies at the same time losing 

 their characteristic arrangement. 

 The cell-wall then opens between 

 the two protuberances, and the 

 whole of the protoplasmic con- 

 tents of one of the two cells 

 passes through the connecting 

 tube thus formed, and glides 

 slowly through it into the other 

 cell-cavity, coalescing with its pro- 

 toplasm-mass. After complete 

 union the combined protoplast is 

 again ellipsoidal or spherical, and 

 scarcely larger than each of the 

 two before coalescence, further 

 expulsion of water and conse- 

 quent contraction having taken 

 place. In some species of Zyg- 

 nema the zygosperm is formed, 

 not in either of the conjugating 

 cells, but in the connecting tube, 

 as in the Mesocarpaceae. In Sirogonium no connecting tube is formed, 

 but conjugation takes place by genuflexion ; the two filaments are brought 

 into contact by a knee-like bend in each ; at the point of contact the 

 adjacent cells of the two filaments are placed in communication by the 

 disappearance of the cell-walls, and a zygosperm is formed in one of the 

 two cells by the coalescence of the two protoplasts. 



A second mode of conjugation, not nearly so frequent, is the lateral, 

 between two contiguous cells of the same filament. Protuberances 

 formed near the adjacent ends of contiguous cells bend towards one 

 another till they meet ; the cell- wall between them then disappears,: 



FIG. 237. Zygnema pecti- 

 natnm Ag., in conjugation 

 (x 100). (After Cooke.) 



FIG. 238. Spiro- 

 gyra be His Hass. ; 

 lateral conjuga- 

 tion ( x 100). 

 (From nature.) 



