226 NATURAL HISTORY OF BHOMPSTON POND. 



action of the several cilia. Comparatively few specimens of 

 this plant were taken. 



The well known Volvox globator (fig. 92) was living in the 

 pond in July, 1911 ; but its whereabouts cannot be depended 

 upon, nor can it be easily kept in a micro-aquarium. It is a 

 hollow spherical colonial form of about l-50th in. in diameter, 

 the individual protoplasts of which are seen to possess an eye- 

 spot and two flagella each (fig. 93). The protoplasts are joined 

 together by exceedingly fine protoplasmic threads. The 

 sphere revolves as it slowly swims along by the united efforts 

 of the very numerous flagella. The large green non-motile 

 egg-cells arise from single cells, while other single colonial cells 

 give rise to yellow ciliated spermatozoids. Often the colonies 

 are seen to contain smaller parthenogenetic colonies (fig. 92a) 

 revolving within the large sphere. 



Pediastrum, another pretty little colonial form (fig. 94), was 

 found in the clear water. It is a green plate of cells, the 

 component parts of which are arranged around a central cell 

 in two concentric circles. The peripheral cells each bear two 

 processes, giving the whole a stellate appearance. The entire 

 cell colony is about 1 -250th in. in diameter. 



Oedogonium (fig. 95) consists of a filament of cells, each of 

 which contains a large single chloroplast (a) with pyrenoids. 

 The filament exhibits intercalary growth (b). The protoplasm 

 of a cell is sometimes seen to have rounded up and formed a 

 zoospore provided with a crown of cilia. After swimming for 

 a while it settles down, and, losing its cilia, it forms a cell wall 

 and grows into a filament. Stages of the sexual reproduction 

 may be met with, e.g., oogonia (c). 



Coleochaete (fig. 96) is a small disk-shaped green alga, 

 composed of rows of radiating cells occasionally branching 

 dichotomously (fig. 97). These rows of cells are joined with 

 one another laterally to form the plate. This thallus gives 

 rise to a flask-shaped oogonium, which, after fertilisation, 

 produces a spherical zygospore (fig. 96a) enclosed in envelop- 

 ing cells, the pseudo-parenchyma. The zygospore divides 

 into sixteen cells, each of which produces a swarm-spore. 



