200 MICRO-ORGANISES AND FERMENTATION. 



protoplasm, and growing towards each other, form club -like 

 swellings, and come in contact at their free ends, which flatten 

 out (5). Each of the branches is then divided into two cells 

 by a septum, and the end cells, which are in contact (the 

 conjugating cells), coalesce by dissolution of the original 

 double wall which separated them. The two conjugating 

 cells are either equal in size, as in Mucor Mucedo, or unequal, 

 as in Mucor stolonifer. The new cell thus formed the zygo- 

 spore (6) quickly increases in size and expands to the shape 

 of a ball (in Mucor stolonifer to the shape of a barrel), after 

 which the wall thickens, and becomes stratified ; externally, 

 it is dark in colour and covered with wart-like excrescences. 

 These outer layers are very resistant to the action of acids. 

 The contents possess an abundance of reserve substance (fat). 

 Cases occur where the zygospore develops from a single cell 

 without conjugation, and occasionally such a cell is formed 

 at the tip of one of the mycelial hyphse. The zygospores are 

 generally only able to germinate after a long period of rest ; 

 the germ tube, after bursting the outer layers, quickly develops 

 sporangia as described above (7). Thus, in the zygospore we 

 find a resting stage of the plant, an organ which by its structure 

 enables the mould to preserve life during periods unfavourable 

 to growth. 



These spores are usually formed only on the surface of the 

 substratum with free access of air. 



The conditions on which the formation of zygospores 

 depend have been exhaustively investigated by Blakeslee, 

 who has shown that, in the case of the majority of species 

 examined, it was essential that the conjugating hyphae should 

 belong to different individuals. In the case of M. Mucedo it 

 is necessary that these individuals should be derived from 

 spores originating from different sporangia, otherwise no 

 zygospores are formed. In some species a difference could 

 be detected in the structure of the two individuals which 

 formed the zygospores. In a minority of cases the zygospores 

 were, however, formed from one and the same mycelium. 

 Temperature affects their formation ; thus in M . Mucedo 

 they were produced at ordinary room temperature, but not 

 at 26-28 C. 



