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The microscope shows at a glance whether our drop contains a 
spore, or several, or none; in the latter case we repeat the drop- 
making process until a drop is obtained which contains ove spore 
only. This result achieved the cover-slip is held in place by 
running in melted vaseline and wax, or any other suitable 
cement, and the culture is ready. 
The drop is now placed under a microscope in such 
a position that the one spore is in focus: a careful 
drawing is then made, the time and temperature, as 
well as the distinguishing mark of the culture noted, 
and each stage of the germination, &c., observed as necessary. 
It is, of course, in the study of the true parasitic fungi that the 
greatest difficulty in obtaining pure cultures must be expected, 
and in most cases no attempt has been made to overcome the 
apparently insuperable obstacles which obtrude themselves. 
For, let us see what a pure culture of a true parasite involves. 
As, before, we have to ensure a pure sowing of the spores of the 
fungus, and since such spores are in the ordinary course of events 
obtained from the host-plant, growing in the open and exposed 
to all kinds of wind-borne intruders, it is obvious that no such 
pure sowing is usually possible. 
In the second place, we have to ensure a pure culture of the 
host-plant, and since this is commonly employed in the form of 
a seedling or pot-plant, growing in ordinary soil in a green- 
house or even in the open, no question of freedom from acci- 
dental infections or impurities can be entertained. 
And, in the third and last place, we must put the conditions 
of the infected plant itself after inoculation. | However carefully 
we cover up the specimen—even if as in some recent experi- 
ments by Eriksson, Klebahn and others, we place tubes or glass 
cases guarded with cotton-wool over them—we have no 
guarantee of purity of culture of either host or parasite. 
Of course, in such infection experiments we rely on the follow- 
ing two principal factors in the results—(1) That infection occurs 
at a definite spot which we have marked beforehand as that 
where the sowing of spores was made on inoculation; and (2) 
that numerous repetitions of the infection—or attempts at such— 
yield consistent results, positive or negative as the case may be. 
There arise questions, however, which make it highly desir- 
able that pure cultures of both host and parasite should be 
obtained if possible, and I have recently been so fortunate as to 
carry into effect pure cultures of this kind even in the case of 
such exquisite parasites as the Uredinee. 
It is quite possible, as I have shown, to sterilise the “seeds ” 
of grasses and germinate them in large glass-tubes on a plug of 
cotton-wool saturated with a suitable mineral solution, the orifice 
