SPORE FORMATION l6l 



secretes a wall upon its surface. Directly the proto- 

 plasm of the sporangium breaks up, its turgidity, which 

 has been maintained by the osmotic pressure in the 

 vacuoles against the semi-permeable cytoplasm sur- 

 rounding them, is destroyed, and owing to the osmotic 

 pressure being still maintained in the sporangiophore 

 on the other side of the cross wall, this is pushed into 

 the cavity of the sporangium and pressed against and 

 into the mass of spores (Fig. i6, b). This pressure 

 squeezes the spores against the wall of the sporangium ; 

 and eventually, with the progressive drying of the 

 wall by evaporation (since it is no longer in contact 

 with the protoplasmic water), the wall becomes brittle 

 and is burst, the spores being scattered into the 

 air as a fine dust (Fig. i6, c). 



The spores float in the air and are easily carried 

 about by slight currents. The air of rooms and of 

 towns generally contains innumerable spores of Mucor 

 and other moulds, as well as bacterial cells and spores, 

 and these gradually settle on all objects when the air is 

 quiet. Thus any suitable organic substance, provided it 

 is moist, will develop moulds or colonies of bacteria, 

 or both, because the spores will germinate (or the 

 bacterial cells grow) on such a substance from which 

 they can get suitable food. The protoplasm of the 

 spore absorbs water and food, the wall of the spore 

 bursts, and the protoplasm, covered by a new delicate 

 wall (Fig. i6, d), is pushed out, grows and branches to 

 form a new Mucor plant. 



Only the air of regions very remote from human 

 dwelhngs, and from the substances on which moulds 

 and bacteria thrive, is comparatively free from spores, 

 and is in that sense pure. The purity of the air in 

 this sense can be measured by the number of mould 



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