208 DR. COHN, ON THE DEVELOPMENT OF 



would be seen to precede the outbreak of any morbid pheno- 

 menon, and in the latter, the reverse would be observable. 

 Other influences of a more general, external, chemical, 

 physical, or it may be, cosmical nature, must, doubtless, 

 concur, to render the disease, however produced — truly 

 epidemic — but these are not now the subject of inquiry. 



The decisifm of the question as above stated is obviously 

 extremely difficult, if not wholly impossible, in the higher 

 plants, owing to the complexity of their structure, and the 

 inability we labour under of tracing microscopically the entire 

 course of the disease, from its first appearance to its termina- 

 tion. 



An observation, therefore, which Cohn states, he made in 

 the year 1852, of a disease attacking unicellular plants, is of 

 the greatest interest ; he observed a sort of epidemy to break 

 out among some Desmideae which he had kept in a flourish- 

 ing condition for some time. The consequence was, that the 

 CAosteria^ especially, were nearly all destroyed with great 

 rapidity. He discovered that the cause of this remarkable 

 epidemic was a peculiar microscopic plant, which attached 

 itself to healthy Closteria, living at their expense, and con- 

 suming or destroying their living contents, and thus killing 

 sthem. The unicellular nature of the victim and of its de- 

 troyer allowed the whole proceeding to be observed tlirough- 

 out all its stages ; and thus, at any rate, 07ie certain fact was 

 established with respect to the significance of epiphytes in 

 epidemic diseases. 



The matter is thus described : — " In the beginning of 

 April he observed upon the dead Closteria of every species, 

 splierical vesicles of very various dimensions, the smallest 

 being scarcely 1-300", and the largest more than 1-50'" in 

 diameter. These vesicles were filled with opaque, fine-gra- 

 nular but colourless contents. They were seated either 

 singly, or in larger or smaller numbers upon the cell-wall ; 

 on some Closteria as many as twenty might be counted." 

 The granular contents of the vesicles were seen to become 

 gradually transformed into motile spores or ' swarm-cells,' 

 which escaped through openings or perhaps ruptures of the 

 parent vesicle, and moved about very actively in the water by 

 means of a single cilinm. When all had escaped, the vesicle 

 was left as a colourless, hyaline membrane. 



The spores themselves resembled in shape the minute 

 monades, and equally resembled the 'swarm-cells' of Achlya 

 prolijera, but from which they differed in their very remark- 

 al)le motility. From this, and from their rorresjiondence in 

 form and size, Cohn regards it as probable tl)at those Chytri- 



