Researches into the Nature of the Potato- Fungus. 
245 
The mycelium of the Peronosporece grows in the living tissue 
of its host, and with the greatest vigour when the foliage and 
flowers are being rapidly produced in the summer ; this to some 
extent determines the principal season of the growth of these 
fungi. At the same time, it should be remembered, that the 
mycelium of particular kinds may in the case of perennial 
plants also continue to live in and along with the perennial 
parts, and annually to spread from them to the parts developed 
in summer, and so extend itself along with these, as, for 
instance, Peronospora Ficaria; in Ranunculus Ficaria, P. Ru- 
inicis in the sorel, Rumex acetosa. 
The tubers of the mycelium grow chiefly between the cells of 
the tissiues of the host-plant, not only touching the cells, but in 
most species also pushing their small branches, which I have 
before called suckers, into the interior of the cells. 
In certain species, which do not concern us here, the coni- 
diophores are formed only on very special regions of the host ; 
in most species they may grow wherever there is mycelium. 
They are produced when the mycelium passes out from the 
tissues of the plant into the air, that is, with rare and casual 
exceptions, upon the surface of the plant, particularly the foliage. 
In Cystopus they form thick layers under the epidermis, which 
they burst open by the increasing accumulation of conidia. In 
Peronospora and Phytophtliora they penetrate the epidermis into 
the air either singly or in small bunches, mostly through the 
stomata ; in parts where these are absent they pierce through the 
cells of the epidermis. The mature conidia fall from their pedi- 
c els, and being light bodies are readily carried away by currents 
of air or by minute animals. As soon as conidia are mature 
they are ready to germinate, and do indeed germinate when sur- 
rounded with moisture, and form zoospores when they are placed 
in water. The germinating spores, when they find a suitable 
host, at once penetrate the epidermis into the interior of the 
plant, and grow into new mycelium, which soon again pushes 
into the air and forms more conidia. A short time and a small 
quantity of water are all that is needed for the germinating of 
these spores and their penetrating the host plant. A single 
damp night, or a short shower, furnishes the necessary conditioris. 
After a few clays the new mycelium sends out its first conidia, 
and these continue to be followed by others for weeks. These 
phenomena explain why it is that the fungus in question is able 
to spread itself in summer rapidly over large districts and attack 
many plants. The conjdia are the organs of this propagation. 
They do not retain their vitality and power of growth for more 
than a few weeks at most. That they remain alive through tbe 
winter has in no case been observed. 
