44 Minnesota Plant Life, 



an extent that in the dust of the air in every living' room in Min- 

 nesota, hundreds of such spores are already floating. It is nec- 

 essary, therefore, only to dip the slice of bread in water and set 

 it aside, for the spores which have fallen upon it in the process 

 to begin their development. It would be a mistake to suppose 

 that moulds originate spontaneously. A mould plant can no 

 more come into existence without the cooperation of some 

 mould-spore than could an oak-tree without the assistance of an 

 acorn. It is because of the presence almost everywhere of incal- 

 culable numbers of mould spores floating invisibly in the atmos- 

 phere that this seeming spontaneity of development impresses 

 one. There are, however, many places where bread will not 

 quickly mould if set out in a saucer, for example, if carried to 

 some high mountain top where the air is pure and free from 

 spores, or if exposed in a chamber which has been purified and 

 sterilized by a spray of carbolic acid. 



The black moulds have a breeding habit reminding one of the 

 pond-scums. Two of the white threads close together or touch- 

 ing each other, may develop little side branches the ends of 

 which blend and gradually convert themselves into a black 

 fused-body with very much the character of a fecundated egg 

 and capable of growing into a new mould thread. 



Other kinds of moulds. There are several other varieties of 

 moulds belonging to this group of algal-fungi, but the well- 

 known blue-moulds, or green-moulds, with their verdigris col- 

 ored fruit-bodies are classified in a higher group. Some of the 

 moulds have curious habits. One, called the pill-throwing 

 mould, produces a mass of spores upon the end of a filament, 

 then underneath this mass there develops a swollen cell in which 

 pressure is exerted, so that after a time the mass of spores is 

 shot off into the air by the explosive mechanism of the stalk-cell. 



Moulds on moulds. Still another mould has the peculiarity 

 of attaching itself to the vegetative body of .the black mould. It 

 lives as a kind of mould-louse, extracting its nutriment from the 

 body of the larger and more vigorous black mould. A plant 

 which thus fastens itself upon another living creature and 

 absorbs nutriment from it to the injury of the host is called a 

 parasite. 



