Zygnemacece 125 



cell is usually thicker and shorter than the part developed from 

 the male cell. The female cells frequently become much swollen 

 after the fusion of the gametes. 



On the examination of a large number of conjugated examples 

 of either Spirogyra or Zygnema one feature must impress itself 

 even on a casual observer, namely, that "the direction of conjuga- 

 tion is clearly governed by some physiological law, the movement 

 of protoplasm between the two filaments almost invariably taking 

 plnrc in one direction only, so that one of the two conjugating 

 filaments is entirely emptied, while the other is filled with zygo- 

 sperms 1 ." Thus, the cells of one filament usually become all of 

 the same sex. 



Two filaments are generally concerned in an example of scala- 

 riform conjugation, but three, four, five, six, or more, are occasion- 

 ally seen (vide fig. 49 A). Such are mostly cases of polygamy or 

 polyandry, and statistics seem to show that the former is the more 

 frequent. 



Lateral conjugation, or conjugation between adjoining cells of 

 the same filament, is frequently observed, but it is much scarcer 

 than scalariform conjugation and may be regarded as exceptional. 

 It is more often met with in Spirogyra than in Zygnema and is 

 especially frequent in some of the smaller species, such as S. 

 tenuissima (Hass.) Ktitz., 8. inflata (Vauch.) Rabenh., and S. 

 varians (Hass.) Kiitz. Cross-conjugation, in which some cells of 

 the filaments are male and others female, and therefore in which 

 perfectly normal zygospores are found in both filaments, is exceed- 

 ingly rare. In both lateral and cross-conjugation, as was mentioned 

 in the general remarks under the order Conjugate, a differentia- 

 tion of sex must take place amongst the cells of the same filament, 

 some becoming male and others female. 



Sometimes certain irregularities are 'met with in the conjuga- 

 tion, and these are no doubt largely due to the increased functional 

 activity of all the cells of the filaments which is such a striking 

 accompaniment to conjugation. Zygospores are occasionally ob- 

 served which have been produced by the coalescence of three 

 gametes, two male and one female 2 , but attempts of this nature 



1 Bennett and Murray, 'A Handbook of Cryptogamic Botany,' London, 1889, 

 p. 266. 



2 West in Notarisia 1891, vi, t. xiii, f. 13 ; Borge in Bihang till K. Sv. Vet.- 

 Akad. Handl. Bd 17, 1891, no. 2, t. i, f. 2; W. & G. S. West in Ann. Bot. xii, 

 1898, t. v, f. 66. 



