MOULDS. 197 
the cells is dissolved, animalcules or mites make their appearance, till at last 
the whole becomes a loathsome mass of putrescence. 
The form of the mould itself is represented (fig. 3) as 
exhibited under the microscope, with the nodose swellings of 
the branches and spores attached to the tips ; these spores are 
filled with a granular mass, from which, as hereafter described, 
zoospores are produced. These spores are in themselves 
capable of reproducing the mould. The branching dendroidal 
threads of this fungus proceed from a creeping mycelium or 
spawn of entangled filaments which interpenetrates the matrix 
upon which it establishes itself. From these threads are also 
produced, though more rarely, a secondary kind of fruit, which 
is spherical and larger than the bodies borne on the tips of the 
threads. Under the name of Artotrogus this fructifying myce- 
lium has been described as another species of fungus ; but it 
is now admitted to be only a stage or condition of Peronospora. 
All the species of this genus noticed in this paper are subject 
to this condition, that is, under favourable circumstances, all 
are liable to produce secondary fruit from the myceloid fila- 
ments. This is not the only instance, even amongst the Fungi 
found upon the potato, of the same parasite under different 
phases having been regarded as a distinct species. 
During the year 1861, Dr. de Bary published an account* 
of the discovery by him of a third mode of propagation in the 
potato mould, and that the most common of any. This method 
is by means of zoospores. The spores produced on the tips of 
the branchlets are certainly capable of germinating when 
placed in favourable conditions ; but, according to M. de Bary, 
important changes more frequently take place in the granular 
matter with which they are filled, and bodies resembling small 
Infusoria are produced, which move about actively by means of 
two long filaments. These active zoospores, or insect-like 
bodies, are by no means restricted to the potato mould, but 
have long since been met with in many of the lower crypto- 
gams. As a result of this method, and the rapidity with which 
the zoospores attain perfection, the number of agents for the 
propagation of the disease, produced in one season, seems 
almost past belief. De Bary calculates that one square line of 
the under surface of the leaves is capable of producing 3,270 
spores, each of which yields at least six zoospores, sometimes 
double that number; thus we have 19,620 reproductive bodies 
from that small space. The mycelium from the zoospores is 
capable of penetrating the cellular tissue in twelve hours, and 
when established there, it bursts through the stomata of the 
* Die gegenwartig herrschende Kartoffelkrankheit, ihre Ursache und ihre 
Verhutung. Von Dr. A. de Bary. Leipsig, 1861 . 
