62 
MILDEWS, RUSTS AND SMUTS 
exposed to the weather, and in the spring it would be well 
to place a few spores in a hanging-drop method of culture, 
to ascertain whether the spores are capable of germination, 
if so, the prom3^celial tubes, bearing promycelium spores, 
should be produced in about twenty-four hours. If the 
teleutospores germinate freely, they may be placed on the 
leaves to be infected. 
BIOLOGICAL SPECIES 
Species, as generally understood, depend entirely on 
morphological, that is, structural characters, but in addi¬ 
tion to such species, there occur in the fungi, certain forms 
that have been termed biological or biologic species. Such 
forms do not differ from each other in structure, conse¬ 
quently cannot be separated from each other by structural 
characters. Biological species appear to have originated 
somewhat as follows : a given species of fungus originally 
grew on several different host-plants, but by degrees, those 
growing on one particular kind of plant became so accus¬ 
tomed to that host-plant, that they were unable to grow 
on any other kind of host-plant. During this process of 
becoming confined to one particular kind of host-plant, 
the fungus has undergone no structural change whatever ; 
but, on the other hand, it has undergone what is called a 
physiological change ; in other words, it has undergone 
such a change that it can only live on the food supplied, 
in addition to the general conditions essential to the well¬ 
being of the fungus, furnished by a particular host-plant. 
This is in reality the result of habit, or an acquired property, 
created perhaps in the first instance by accidentally growing 
on the same host-plant for several seasons in succession. 
As an example, Erysiphe graminis is parasitic on wheat, 
barley, oats, rye, and several wild grasses, but experiments 
have proved that the form of E. graminis on each of these 
host-plants, although not differing in the least in structural 
details, has become specialised or confined to its own parti¬ 
cular host-plant, and the spores produced by the fungus 
on one host-plant cannot infect any other host-plant on 
which other biologic forms of E. graminis are parasitic. 
Economic aspects 
Some countries, England for instance, do not furnish 
statistics as to the loss caused to cultivated plants by fungi. 
The United States, Germany, Australia, etc., however, 
do so, and from most carefully compiled reports it is cal- 
