PESTIFEROUS FUNGI. 
385 
elementary knowledge of the fungi to determine whether the pests he 
has to deal with are of this nature is already in possession of the power 
to treat them effectually. Even the very crude method of picking off 
the diseased leaves and burning them will limit the area of infection. 
More important and more destructive are the endophj/tnl para- 
sites, which originate within the tissues of the host plants, and only 
manifest themselves externally, when it is too late to save the plants. 
The “rot moulds” are of this kind, such as the potato mildew, American 
vine disease, tobacco mildew, and man} r other devastating pests. They 
are called “ rot moulds” because of the rotting of the leaves and stems 
subsequent to their attacks. Their scientific designation is Perono- 
sporacece , and they have the habit and appearance of white moulds, 
but are parasitic on living plants. Here again it is of the utmost 
importance to know something of their life history and methods of 
reproduction before they can be combated with success. The mature 
mould, whert it appears on the surface of a diseased plant, produces a 
profusion of spores or eonidia. Each conidium is an elliptical colour- 
less body, having a thin outer coating of membrane with fluid contents. 
These contents soon become granular, and at length collect at three or 
four centres, which condense and soon become distinctly separated 
from each other by the growth of a special envelope. Ultimately the 
membrane of the mother cell is ruptured, and the three or four 
smaller bodies, which have been differentiated in its interior, escape, 
each one furnished at one extremity with a pair of delicate movable 
hairs, by means of which these little bodies, now termed zoospores , can 
swim actively in any thin film of moisture upon which they may fall. 
Possibly this film may be upon the leaf of a foster plant. In a short 
time all motion ceases, and the zoospores come to rest, the pair of 
delicate cilia arc absorbed, and a germinating thread is produced, the 
point of which seeks out and enters at one of the stomata of the 
sustaining plant. Having once obtained an entrance, the thread 
grows vigorously, and a little mass of threads, called a mycelium , is 
soon developed within the tissues, capable of spreading itself through 
the plant which it has infected. In the next stage we discover that 
this mycelium has developed erect branched threads, which pass out 
through the stomata again into the external air, sometimes singly, 
sometimes in tufts. These are the fertile threads of the mould, which 
soon produce a single conidium at the tip of each of the branchlets, 
just like the original conidium whence the zoospores were developed. 
When fully matured each fertile thread produces a score or more of 
these eonidia, which fall away when ripe, and then undergo transfor- 
mation into zoospores, ready and active, prepared to pass through the 
same stages again, and indefinitely multiply the pest. This history 
represents the ordinary conidial fructification of the mould, by means 
of which it is passed from leaf to leaf, and from plant to plant, until 
the whole area is affected. How many of the minute eonidia may be 
transported to a considerable distance by a breath of wind it is 
impossible to say, but it is known that they may be carried to any 
spot where there is sufficient moisture for the eonidia to be differen- 
tiated into zoospores, and afterwards come to rest and germinate. 
This process takes place in summer and autumn, but there is yet 
another means by which the pest is disseminated in spring. 
2 A 
