CON JUG A T.^ 267 



female are both longer and broader than those of the male filament ; 

 and the contraction of the protoplasm has been observed to begin 

 earlier in the male than in the female filament. It is also stated that 

 the protuberance from the female cell is shorter but broader than that 

 of the male cell, the latter fitting into the former as into a socket. The 

 chief argument against the sexuality of the filaments is the occurrence 

 of lateral conjugation ; and when this takes place a sexual differentiation 

 can be assumed only of the individual cells and not- of the filaments ; 

 but that there is some differentiation of this kind would appear from the 

 fact that when lateral conjugation takes place in a group of four cells 

 the zygosperms are formed in the two centre cells, which may be 

 regarded as female. The phenomena may then be compared to those 

 in Sphaeroplea. Bessey states {in lit.) that scalariform and lateral conju- 

 gation may sometimes be seen in different parts of the same filament. 

 The female filaments are, as a rule, very much more abundant than the 

 male ; and it is not uncommon for conjugation to take place between 

 one male and several female filaments, while the reverse is at all events 

 much more rare. Occasionally one cell will conjugate with two others^ 

 the zygosperm being then the product of one female and two male cells. 

 Several instances are recorded of hybridism between two different species 

 of Spirog}Ta. Parthenogenesis, or the formation of parthe?ios_per??is 

 capable of germination, and in all respects resembling zygosperms, but 

 formed out of the contents of a single cell without any previous process 

 of conjugation, is also stated to occur. 



The genera included in the Zygnemacege with the above characters 

 are Zygnema (Ktz.), Spirogyra (Lk.), ]\Iougeotia (de By.), Sirogonium 

 (Ktz.), and Zygogonium (Ktz.). Several species of Spirogyra and 

 Zygnema are among the commonest of fresh-water Algse in both stagnant 

 and running water, forming dense bright green masses, often with a 

 slimy feel, owing to the well-developed mucilaginous sheath in which 

 each filament is enveloped. While conjugation is in active progress, 

 which is mostly in the early summer, the filaments of Spirogyra assume 

 a dull green or even brown colour, easily recognised by the naked eye. 

 The other genera are more frequent in moor pools. 



Literature. 



Pringsheim — Flora, 1852, pp. 465 et seq. 

 . Cleve — Monografi Zygnemaces, 1868. 

 Hofmeister — Wiirtemb. naturw. Jahresheft, 1874, p. 21 1. 

 Overton— Ber. Deutsch. Bot. Gesell., 1S88, p. 68. 



(For fuller bibliography see Bennett, Journ. Linn. Soc, xx. , 1SS4, p. 430)- 



