84 PRACTICAL BOTANY. 



disk-shaped contents ; the disk breaks up into a large number of 

 rod-like bodies arranged like a bundle of cigars. These may 

 under favourable circumstances, be seen to separate, and escape 

 from the antheridium into the cavity of the ccenobium as spindle- 

 shaped antherozoids having two cilia attached laterally. 



ii. The oogonia are easily recognised as enlarged flask-shaped 

 cells which project into the cavity of the coenobium. 



iii. The result of fertilisation is the oospore : these may be 

 seen in considerable numbers in a single cosnobium : they are of 

 spherical form, and show a thick cell- wall, developed inside that of 

 the oogonium : the wall consists of an epispore, a firm membrane, 

 with numerous superficial spines, and an endospore, which is a 

 smooth layer. The germination has been observed in V. minor 

 to result in the formation of a new coenobium by a process similar 

 to that in the vegetative reproduction : the same is presumably 

 the case in V. globator. 



If material be at hand, observations should also be made on 

 Pandorina, the freely-swimming coenobium of which is simpler 

 in structure than that of Volvox, consisting only of sixteen cells. 

 The sexual process consists in the conjugation of two freely- 

 swimming, and similar swarm-cells. 



PLEUROCOCCUS VULGARIS, Meneg. 



This is the organism which is universally to be 

 found forming a bright green pulverulent incrustation 

 on the bark of trees, wooden rails, &c. If a small 

 quantity of it be mounted in water, and examined under 

 a low power, it will be seen to consist of cells with 

 green-coloured contents ; occurring sometimes solitarily, 

 but more frequently in groups of two, or four, or even 

 larger numbers. Examination under a higher power 

 will show that they multiply by division, and that 

 the resulting cells tend to round themselves off. Note 

 especially that in groups of four or more cells, a 

 splitting of the cell-wall at the centre of the group 



