Conjugatce 115 



by the formation of resting-cells with thick walls, which have 

 received the name of ' cysts.' These are capable of surviving the 

 winter. In both the Zygnemaceae and Desmidiacese aplanospores 

 are sometimes formed. Zoogonidia are entirely absent from this 

 order of green Algae. 



Sexual reproduction takes place by the conjugation of isogamous 

 gametes and the formation of a zygospore. As the gametes are 

 devoid of cilia and therefore non-motile, they are known as aplano- 

 gametes. The ordinary vegetative cells become the gametangia, 

 usually without change of form, and only one gamete arises from a 

 gametangium, the entire cell-contents of which are generally 

 utilized in its formation. In the Desmidiaceae (with a solitary 

 exception) the gametes are set free and conjugation takes place 

 outside the gametangia, but in the Zygnemeae the gametes unite 

 either within one of the gametangia or within the connecting-tube 

 which joins them. In these groups the zygospore after a period 

 of rest forms a new gametophyte, but in the Mesocarpeae the 

 zygospore immediately forms a rudimentary sporophyte with one 

 spore (a carpospore) ; the latter undergoes a long rest. 



Much has been written concerning the sexuality of the Con- 

 jugatae, and much of the evidence which has been brought forward 

 of late years indicates that sexuality of a low type does exist. 

 This sexuality is less marked in some Conjugates than in others, 

 and least of all in the Desmidiaceae. The cells in a filament of 

 Zygnema or Spirogyra need not necessarily be considered as. all of 

 the same sex. It is quite possible that they have no sex until just 

 prior to conjugation. There is no visible change on the conversion 

 of an ordinary vegetative cell into a gametangium ; the change is 

 a physiological one which most probably takes place immediately 

 antecedent to conjugation, and the formation of a male or a female 

 gametangium may depend upon restricted local conditions. Taking 

 this into consideration, it is no more surprising to see both male 

 and female gametes produced from the cells of a single filament 

 than to see a filament which gives origin to gametes of one sex only. 

 It has been shown that a strictly filamentous condition is of no 

 essential importance to the life of the Conjugate, and also that the 

 functional activities of the cells of a filament are greatly increased 

 during conjugation, even in those cells which take no part in the 

 actual conjugation 1 . Thus, there is no reason why the physiological 

 1 West & G. S. West, ' Obs. on the Coiij.,' Ann. Bot. xlv, 1898, pp. 30, 36, 37. 



89 

 A 



