314 FIRST FORMS OF VEGETATION. 
four minute spores supported on tiny stalks. It 
is by these spores which become detached when 
ripe that the plant is propagated. When a small 
fragment of a ripe gill is placed on the glass slide 
of the microscope, in a drop of water, the spores 
will detach themselves from the gill and float 
freely on the water ; or even if a whole mushroom 
be laid on a sheet of paper, it will often leave 
behind its spores in the form of a thin impalpable 
powder. These spores are so very minute, that 
many thousands of them are required to make a 
body the size of a pin-head ; and they are capable 
of enduring a temperature at least equal to that of 
boiling water, as was satisfactorily proved a few 
years ago when the barrack bread in Paris was 
affected with mould, which was in active growth 
almost before the bread was cold. They are also 
of different colours, being white, rose-coloured, 
brown, purple, and black; and this variety of 
spore-tints affords a ready test for grouping 
species of Agarics, which by a little practice will 
become easy. The spores and sporidia of fungi 
have a singular tendency to appear in definite 
numbers, either in twos, fours, or multiples of 
four. Every observer is struck indeed with the 
quaternary arrangement which prevails in all 
cryptogamic plants, while few facts are more 
curious than that the number four should prevail 
