330 Conjugatae 



The sexuality of the Conjugate is of a low type and is much less evident in some 

 Conjugates than in others. In the Desmidiaceo; the fusion of the gametes is usually a 

 mere gamogenesis of equal and similar reproductive cells. In Spirogyra and many species 

 of Zygnema the actual fusion of the gametes takes place within one of the gametangia, 

 which may be to some extent differentiated, either by its swollen character or by the 

 width of its contribution to the conjugation-tube. This gametangium is usually regarded 

 as female in contrast to the emptied one, which is designated the male gametangium. 

 Thus, although the gametes are morphologically indistinguishable, they must be physio- 

 logically differentiated, and there is often an obvious morphological differentiation of the 

 gametangia. In the conjugation of most species of Spirogyra it is usual for one filament 

 to be completely emptied while the other is filled with zygospores, the physiological 

 differentiation of gametes being uniform in each filament; but in lateral conjugation and 

 the rarer instances of cross-conjugation physiological differentiation of the gametes occurs 

 in the same filament. 



In Sirogonium and Temnogametum the gametangia are clearly differentiated 

 from the vegetative cells, being short cells specially cut off for reproductive 

 purposes. 



All the Conjugates are inhabitants of fresh water and, so far as has been 

 ascertained, in no single instance has a Conjugate succeeded in adapting itself 

 to a marine life 1 ; moreover, the majority of species live only in still water, 

 the few exceptions including Spirogyra fluviatilis Hilse, Sirogonium sticticum 

 Kiitz. and some Desmids. Oltmanns' statement that the Conjugate are with 

 few exceptions cosmopolitan is a very erroneous one, since the Conjugate, 

 and especially the family Desmidiacea?, show more decided geographical 

 peculiarities than any other Green Alga? (W. & G. S. W., '07 ; G. S. W., '07 

 and '09). 



Concerning the classification of the Conjugata? there have been various 

 suggestions in recent years, but many of the proposed changes are of a rather 

 speculative character and can scarcely be upheld after a careful and detailed 

 consideration of the actual facts. 



It was Kiitzing who first used the group-names Desmidieae and Zygnemacese, although 

 in his Species Algarum ('49) they are found widely separated, their close affinity not 

 having been realized. De Bary ('58) divided the Conjugate into the Mesocarpeae, 

 Zygnemeoe and Desmidieae, but Rabenhorst ('68) recognized only the Desmidiese and 

 Zygnemete. 



Palla ('94) was the first to attempt a complete revision of the Zygnemace0e and he 

 based his groups of the Spirogyraceae, MougeotiaeeEe and Zygnemacese upon the nature of 

 the chloroplasts. Whatever the merits of this classification, and they are certainly great, 

 Palla was in error in placing his Mougeotiacese between the very closely allied Spirogyracese 

 and Zygnemacese. 



W. & G. S. West ('97) set up the Pyxisporete as a sub-family of the Zygnemacese and 



1 Cosmarium salinum Hansg. has been described as living in brackish water, and several other 

 small species of Cosmarium occasionally occur in limited numbers in water in which brackish 

 Diatoms are living. The larger species of Spirogyra may also be cultivated in water of weak 



salinity. 



