EEPEODUCTION OF THALLOGEKS. 767 



existing at the point of contact becoming absorbed, the two 

 cells freely communicate. The contents of the cells then con- 



Fig. 1109. 



^m^s^mn^^M^^^^ 



^ - — ^'^m^^mm 



Fig. 1109.— Two Desmidiaceous AlgEe {Bocidium Ehrenhergii), in process of 

 conjugating:. The contents of the two are seen to be intermingling in 

 the intervening space, and are at present only invested by a primordial 

 utricle. After Ratfs. 



tract into a mass, and iiltimately combine together, either by 

 the passage of the contents of one cell into the other, or by the 

 mixture of the contents of the two cells in the tubular process 

 between them. Under either circumstance, the mixture of the 

 contents of the two cells results in the formation of a resting 

 spore {oospore), which ultimately germinates and becomes an 

 individual resembling its parents. 



2. Impregnation of naked spores or germ-corpuscles hy ciliated, 

 spcrmatozoids. — This mode of fecundation has been observed 

 generally in the Melanosporege and Ehodosporese, and rarely in 

 some of the Chlorosporese. There appear to be two forms of 

 this fecundation : thus, in certain Chlorosporege, the fecun- 

 dation takes place before the spore has separated from its 

 parent; and in the Melanosporesej after both the spore and 

 ciliated spermatozoids have been discharged. 



As an illustration of the first mode of fecimdationviQ may take 

 Vaucheria. It is thus described by Henfrey: — "This plant is 

 commonly propagated by a peculiar kind of zoospore {fig. 833, g), 

 discharged from the thickened end of the filament or its branches. 

 Eut at certain epochs lateral structures are developed at the 

 sides of the filaments, as branch-cells, which become shut off 

 from the main tube by septa ; some of these processes expand 

 into ovate and beaked, or bird's-head-shaped bodies, others into 

 short curled filaments or ' horns.' The former are sporangia, 

 the latter antheridia. Wlien ripe, the antheridia or 'horns' 

 discharge their cell-contents in the form of numerous spindle- 

 shaped corpuscles, moving actively by the help of a pair of cilia. 

 Meanwhile an orifice is formed in the beak of the sporangium, 

 and some of the spermatozoids make their way in, so as to come 

 into direct contact with the cell-contents. This phenomenon 

 is followed by the closing up of the sporangium by a mem- 

 brane, and the conversion of its contents into a fertile resting 



