38 PLANT-LIFE 



Such is the life-cycle of Spirogyra. Let it be noted 

 that the zygotes are formed from the fusion of gametes 

 that, to outward appearance, are similar. Yet one of the 

 gametes shows decided activity in passing out of its 

 cell into the cell containing the other. To put it differ- 

 ently, one gamete stays at home and awaits the visit 

 of the other; one is practically passive, the other active. 

 The passive gamete may be likened to an ovule and the 

 active gamete to the male element which fertilizes it. 

 This idea of the difEerentiation of the sexes is confirmed 

 in two ways : the chloroplast of the developing zygote 

 is observed to be that of the passive gamete, while that 

 of the active gamete is dissolved; and if the processes 

 of conjugating cells are not exactly opposite, that put 

 out by the female cell bends so that it may meet that 

 proceeding from the male. And if the male gamete 

 is prevented from reaching the female, the latter cell 

 remains in its virginal state. In the absence of con- 

 jugation, unfertilized cells may, on occasion, form 

 asexual resting-spores. So in Spirogyra we observe 

 what we have not noticed in our previous subjects — 

 differentiation of sex. 



The genus Zygnema belongs to the same order and 

 class as Spirogyra. The species of Zygnema are recog- 

 nized by the two beautiful, star-shaped chloroplasts, 

 separated by a space containing the nucleus, in each cell. 

 The process of conjugation is similar to that in Spiro- 

 gyra, with the exception that zygotes may be formed 

 in the tubes connecting the conjjigating cells. Zygnema 

 should be looked for in all habitats favoured by Spiro- 

 gyra ; indeed, several species of each genus may occur 

 in one floating mass. 



