60 PLANT-LIFE 



we have the conjugation of sexually undifferentiated 

 gametes, and also the occasional ^production of ap- 

 proaches to egg-cells and spermatozoids. The sexual 

 distinction is not pronounced, but it is sufficiently 

 marked to indicate that full distinction arose from such 

 an intermediate stage by the elimination of intermediate 

 sizes of gametes, only the largest remaining and becom- 

 ing passive egg-cells full of nutriment, while the smallest 

 became specialized into active spermatozoids. 



Within the family Volvocese, which includes Gonium 

 and Pandorina, we have a complete series of forms 

 illustrating the evolution of sex. One would fain refer 

 to a number of interesting and instructive types, but as 

 our space is limited we must be content with a short 

 description of a form in which sex distinction is con- 

 summated. The little whirling globes of Volvox globator, 

 swimming majestically in a :i live-cell " under the 

 microscope, are always the wonder and admiration of 

 the observer. The life-cycle of this remarkable plant is 

 ably illustrated in Plate II., Figs. 8-11. Volvox is found 

 in ponds; it seems to be a very delicate plant, being 

 extremely sensitive to changes of temperature. It is 

 visible to the naked eye, as it averages about a milli- 

 metre (aV inch) in diameter. A person seeing this plant 

 for the first time might be forgiven for thinking it an 

 animal, and would certainly be astounded when told 

 that it was a plant composed of a colony of cells ranging 

 from 15,000 to 22,000 in number. The individual cells 

 are of the Chlamydomonas type; they are united to each 

 other by strands of protoplasm and form a sphere, the 

 bases of the cells pointing inwards, and the bi-flagellated 

 apices outwards. The plant swims, in dignified rota- 



