570 A MANUAL OF BACTERIOLOGY 



in size it changes in colour, becoming bluish -green, though the 

 margin for some time still remains white. From the upper surface 

 of the mycelium delicate aerial hyphse grow upwards, and from 

 the under surface short submerged ones project downwards. 



The hyphse are composed of elongated cells arranged end to end, 

 the cell-walls of which consist of cellulose enclosing a more or 

 less vacuolated protoplasm containing several nuclei. 



The aerial hyphse are unbranched filaments, but as development 

 proceeds the distal ends branch dichotomously, the branches 

 remaining short and nearly parallel to one another, so that a kind 

 of brush is produced. The ultimate branches are known as 

 sterigmata. The ends of the sterigmata become constricted so 

 that little globular masses, the spores, are formed ; this process is 

 repeated until a chain of spores results, the proximal one being 

 the youngest. A spore when placed under favourable conditions 

 germinates, a little bud appearing, elongating, and forming a 

 hypha, just as in Mucor. 



Brefeld, by sowing spores on moist bread, inverting the bread, 

 and examining at intervals, observed a sexual method of repro- 

 duction in Penicillium. Two sets of spiral cells develop on a 

 thick hypha, they intertwine, their contents probably mingle, 

 and from the union or carpogonium a tube-like hypha develops, 

 which becomes surrounded and enclosed by branching hyphse 

 from the mother cell. By further development and thickening of 

 the cell-walls a sclerotium forms ; it is a hard solid body, yellowish 

 in colour, and resembles a grain of sand, the carpogonium being 

 at the centre. If placed in favourable conditions the sclerotia 

 germinate after some time. Two forms of hyphse are produced, 

 one thick, the other thin ; the latter become much twisted. The 

 thick hyphse become branched, and ultimately a number of pear- 

 shaped bodies are produced. The contents of these bodies then 

 become broken up and form spores ; the bodies are known as 

 asci and the spores as ascospores. From the ascospores the 

 ordinary mycelial form again develops. 1 



Aspergillus niger 



Aspergillus also belongs to the Ascomycetes, and representatives 

 of this genus are common on damp and decaying vegetable matter. 



1 See Brefeld, Quart. Journ. Microscop. Soc., vol. xv, p. 342. 



