98 PLANT LIFE. 



green chlorophyll at the broad end ; the pointed end is 

 destitute of chlorophyll, but contains a bright red pigment- 

 spot, and is furnished with two cilia. The zoogametes 

 vary in size, which however is not supposed to correspond 

 to a difference of sex, and as they swim actively about 

 conjugate in pairs, irrespective of relative size. The zygo- 

 spore resulting from the conjugation of two zygogametes 

 becomes surrounded by a firm cell-wall, changes to a red 

 colour, and passes into the resting-stage. On germination 

 the contents of the zygospore escapes in the form of from 

 one to three motile zoospores, which after a short period of 

 activity retract their cilia, secrete a mucilaginous covering, 

 and form the starting-point of coenobia. The present species 

 is interesting, as being the first in which the conjugation of 

 zoogametes was observed. 



Volvox globaior is a favourite object with microscopists, 

 and consists of a minute hollow bright green sphere, which 

 exhibits a rotatory movement in the water, and is in reality a 

 ccenobium, quite visible to the naked eye, and often occurs 

 in immense numbers in still, clear water. The wall of the 

 colony consists of more or less pear-shaped motile cells, 

 having the pointed end towards the periphery and furnished 

 with two slender cilia, which protrude through the envelop- 

 ing mucilage into the water. The great majority of these 

 peripheral cells, which are individually enclosed in a 

 gelatinous sheath, possess vegetative functions only so far 

 as is known, and possess the peculiarity of being connected 

 with each other by minute strands of protoplasm — a 

 primitive form of protoplasmic continuity. Certain cells of 

 the colony distinguished by their larger size are reproductive 

 in function, and by the division of their contents give origin 

 to asexually formed young colonies, which remain enclosed 



