The Freshwater Algae of Africa, 565 



altera versus alteram curvatis, ita ut margo exterior cellulae copulantis 

 • •oiicii \ us sit ; filo altero inter copulationem circo alterum spiraliter 

 torquato ; zygosporis latum tubum copulantem fere replentibus ; 

 rnembranae cellularum copulantium et tubi copulantis lamellis plus 

 minus gelatinosis appositis incrassantur et, ut videtur, integument uni 

 zygosporae faciunt. 



Diam. cell., 18-25 /* ; (list, inter margin, exter. cellularum copulan- 

 tium (/. e. diam. zygosp.), 48-54 /a; crass, zygosp. a latere, 30-36 //, ; 

 crass, membr. zygosp. a latere, usque 5 /*. 



Sample 39 (rather common). 



The appearance of the ordinary vegetative cells of this species and 

 of filaments, not in process of conjugation, is like that of any of the 

 other common species of the genus (Fig. 34, h). There are two stellate 

 chloroplasts, rather near together, with a small round nucleus in the 

 central bridge of protoplasm. The membrane is in no way markedly 

 thickened and there is no constriction between the cells, the outer 

 margins of the threads forming perfectly straight lines. 



The peculiarities arise in connection with conjugation. In a few 

 cases conjugating threads have been encountered in which the wide 

 conjugation-tubes were fully established, without any of the charac- 

 teristic thickenings of the membrane having as yet arisen (Fig. 34, c), 

 but this is certainly unusual. Although my material contained none 

 of the preliminary stages, to judge by subsequent appearances, the 

 met hod of conjugation is a combination of the scalariform type usual 

 in Zygnema and of the geniculate type, characteristic of Mougeotia 

 (Fig. 34, a-d). There is no doubt that definite conjugation-tubes are 

 put out, but at the same time the conjugating cells evidently bend 

 slightly towards one another. This is evidenced by the fact that 

 every conjugating cell was markedly concave on its outer surface, a 

 change which is very striking when, as in Fig. 34, c, an unconjugated 

 cell, which is straight, occurs in the course of a conjugating filament. 

 The conjugation-tube is in general very wide. It may be mentioned 

 here that conjugation of one filament with two or three others was not 

 uncommonly observed. 



A second, most characteristic feature of the conjugation-process, 

 which has led to the specific name given to this Alga, is that the two 

 filaments are always twisted spirally upon one another (Fig. 34, e). 

 This was noticed with unfailing regularity, wherever some length of 

 filaments was involved in the conjugation-process. I have been unable 

 to discover the cause of this phenomenon, but it is obviously a definite 

 character of the species. 



At some stage during the conjugation-process marked thickening of 



