152 PHOTO-MICROGRAPHS. 
break up into spores like those from which the 
little plant originated. (Plate III. Fig. 3.) 
This is the simple history of non-sexual repro- 
duction: a fragment of protoplasm detached trom 
the parent forms a new individual. 
In like manner, in sexual reproduction, little 
masses of protoplasm, — sperm-cell and germ-cell, 
—are detached from each parent, and these coa- 
lesce to form the fertilized ovum from which a new 
individual is developed. The spores of which we 
have spoken are produced in such abundance that 
there is little chance of the extinction of species 
from scarcity of seed. This seed is borne far and 
wide through the air, and whenever conditions are 
favorable for its germination, there it is sure to 
find its way, and we have a growth of Penicillium, 
—blue mould. But this, although one of the 
most common and widely distributed of the micro- 
scopic Fungi, is only one out of a multitude of 
species. Many of these are parasitic upon higher 
plants, where they constitute the pests known as 
mould, mildew, rust, smut, rot (potato-rot), ete. 
The important part which the Bacteria and the 
microscopic Fungi play in the economy of nature 
is a recent revelation of science. Formerly, it was 
supposed that these organisms were present in 
substances undergoing putrefaction or fermenta- 
tion, because the products evolved during these 
processes were suitable for their nourishment. 
Now it is known that these processes depend upon 
the presence of the living organisms, and that the 
most putrescible substances may be preserved in- 
