126 Chlorophycece 



are usually abortive 1 (fig. 49 B). Gerassimoff has observed the 

 conjugation of two female cells with one male cell and the forma- 

 tion of the zygospore by the coalescence of the protoplasm of the 

 male cell with that of one of the female cells, a parthenospore 

 being formed in the other female cell. In those plants of the 

 ZygnemeaB in which the zygospore is formed in the conjugating- 

 tube, conjugation between three cells entails the production of 

 two zygospores, each of which is less than the normal size. Owing 

 to sudden changes of local conditions it frequently happens that 

 the conjugation has been brought to an abrupt termination before 

 the proper formation of the zygospores has taken place. In such 

 cases of interrupted conjugation the spores are apt to be very 

 variable ; sometimes the spore is not of its true form, and at other 

 times one small one is formed in each gametangium or two small 

 ones in the female gametangium 2 . The discovery by Gerassimoff 

 of binucleated cells in the Zygnemese may perhaps afford an expla- 

 nation of some of these irrregularities of conjugation. He states 

 that in the conjugation of binucleated cells parthenospores were 

 sometimes observed. 



Rare instances of hybrids are known, in which conjugation, 

 with the production of zygospores, has occurred between filaments 

 of different species 3 . 



The ripe zygospores possess a thick wall of cellulose which is 

 divisible into three coats, the outer one being cuticularized and 

 the middle one usually exhibiting some form of sculptured mark- 

 ings. The contents of the zygospore often turn red in colour and 

 develop a large amount of oily material. Sometimes the chloro- 

 plasts of the two gametes remain intact in the zygospore ; some- 

 times those of the female remain intact and those of the male 

 disintegrate ; but it is quite a common thing for the chloroplasts 

 to completely disintegrate before the fusion of the gametes. The 

 zygospores undergo a period of rest, surviving the winter or even 

 a period of drought, and they usually germinate after the conju- 



1 W. & G. S. West, 1. c. figs. 67 and 69 ; Schmula in Hedwigia, xxxviii, 1899. 

 (c. fig.) Copeland states that in such a case the nucleus of the abortive male cell 

 is situated against the wall opposite, remote from the conjugating-tube. (Cf. Bull. 

 Torr. Bot. Club, xxix, 1902.) 



2 Rosenvinge in Ofvers. K. Vet.-Akad. Forh. 1883, no. 8, t. viii, f. 111; West 

 in Notarisia 1891, vi, t. xiii, f. 27, 28; Hansgirg in Hedwigia 1888, t. x, f. 6; 

 W. & G. S. West, 1. c. t. v, f. 7480. 



3 Cf. Spirogyra maxima var. iruequalis Wolle Freshw. Alg. u. s. t. cxxxviii, 

 f. 5, 6; t. cxlii, f. 5, 6; W. & G. S. West, 1. c. t. v, f. 70, 71. 



