CONJUGATION. 



[ 205 ] 



CONJUGATION. 



Fig. 139. 



special coats, boccme spores, which escape by 

 the rupture of the conjugated cells. InMou- 

 geotio (fig. 139) 

 there is no cross- 

 branch. The fila- 

 ments become an- 

 gularly bent and 

 inosculate at the 

 angles. A spore 

 is said to be form- 

 ed in each cell 

 here. (See Zyg- 



NESr.\CEJE.) 



Pseudo - spores 

 are sometimes 

 formed in the cells 

 of Zygnemaceae 

 without conjuga- 

 tion, in which case 

 they are barren. 



In the Desmidiacese, 

 seuts a number 



Mougootia genuflt'xa. 



Conjugating filaments. 



Magnified 100 diameters. 



the process pre- 

 of modification?. In the 

 filamentous forms, such as Hyuhtheca and 

 Didymoprimn, conjugation does not usually 

 take place until the single cells of the 

 filaments have become separated ; but in 

 some cases, as in D. Borreri, conjuga- 

 tion of the filamentous groups has been 

 observed, perhaps this occurs in Des- 

 midium also. In Closterium, Penimn, Tet- 

 memorus, Costnariian, Szc, the free cells 

 conjugate in pairs. In ahnost all these 

 cases the mode of union appears to be 

 different from that which is seen in Zygne- 

 maceai ; for the external membrane dehisces 

 more or less completely, so as to separate the 

 parent-cells into two valves, while a deli- 

 cate internal membrane previously lining 

 this is protruded as a sac, to meet its fellow 

 from the corresponding conjugating indivi- 

 dual; these sac-like processes coalesce, and 

 thus the contents of the cell are enabled to 

 mix. In Hyalotheca dissUiens and Penhnn 

 Breh'ssom'i, there is said to be union of the 

 primary or outer cell-coat, as in Zyyncma. 

 The resulting spore or sporangium is mostly 

 formed in the connecting piece (Closfermm, 

 Cosmariitm, Tetmemorus, Hyalotheca) (Pl.lO. 

 figs. 1-3) ; or in one of the cells (^Didyino- 

 prium GreviUii, and perhaps in Desmidium). 

 In Closterium lincntuni it has been observed 

 that the corrugating cells divide completely 

 by constriction of their delicate internal 

 membrane just before conjugation, so that 

 the dehiscent primary membranes emit from 

 each parent individual a pair of little sacs, 

 in close appo.-ition ; and these meeting their 

 fellows, a double or ficin conjugation takes 



place, and a pair of spores or sporangia are 

 formed. A gelatinous investment is secreted 

 around the conjugating sac-like processes, 

 and the spore is generally at first imbedded 

 in an abundant gelatinous coat. (See Des- 



MIUIACE^.) 



In the DiatomacetB there does not appear 

 to be any delicate internal membrane, like 

 that of the Desmidiacese, concerned in the 

 conjugation. The two conjugating indi- 

 viduals, lying near together, become con- 

 nected together by the excretion of a collec- 

 tion of gelatinous substance ; the siliceous 

 coats then dehisce, and the contents of the 

 parent-cells, escaping from the valves, meet 

 between them to unite into a globular mass, 

 which does not become a spore, but gradually 

 acquires the form of the parent. There is 

 no connecting tube here ; only the investing 

 gelatinous matter. In Ilhnantidiwn and 

 Surirella, one new individual is formed in 

 the conjugation (PI. 10. fig. 5) ; in Eunotia, 

 Cucconema, Gomphonema, and Schzonema, 

 the contents of the parent-cells appear to 

 divide traiisversely before extrusion, and 

 thus form a pair of new individuals in the 

 conjugation (PI. 10. fig. 6) (as in the case 

 of the spores of the Closterium lineatimi). 

 A peculiar condition occurs in other genera 

 {Cyclotella, Melosira, kc). which is supposed 

 to be a conjugation of the divided contents 

 of one frustiile. (See Diatomace^.) 



Among the Palmellacese, conjugation has 

 been observed in Coccochloris Brehissonii 

 {Pabnoylosa macrococca, Braun), where two 

 vegetative cells become completely fused, 

 membrane and contents, to form a spore 

 which acquires a firm coat and oily contents, 

 and passes through a stage of rest before re- 

 commencing vegetative development (PI. 7. 

 fig.6,c,c^). 



Fig. 139*. 



Pandorina morum. Conjugation of zoospores. 

 Magnified 300 diameters. 



The zoospores of Pandorina conjugate 

 much as in Coccochloris; they approximate 

 by their apices (fig. 139*, J), become fused (c) 



