MILDEWS, RUSTS AND SMUTS 
3 
PERONOSPORACEAE 
The members of the present family are primitive types, 
and illustrate the earliest attempt on the part of fungi 
to depart from their original aquatic habitat, and establish 
themselves on dry land. Purely aquatic fungi usually 
possess sexual organs of functional value, and lack the 
reproductive bodies known as conidia, so characteristic 
of terrestrial fungi. The ultimate reproductive bodies 
produced by aquatic fungi are called zoospores, because 
they possess the power of spontaneous movement, brought 
about by the presence of cilia, or very slender prolongations 
of the body of the spore, the movements of which enable 
the zoospore to swim in water. Now this method of spore 
dispersion was quite effective so long as fungi lived in water, 
but when they essayed to live on dry land, the possession 
of zoospores alone restricted them to damp localities where 
water was present in sufficient quantity to enable the 
zoospores to migrate from one place to another. This 
difficulty was by degrees overcome by the terrestrial fungi 
evolving a second form of reproductive bodies called 
conidia, suitable for being dispersed by wind. It was this 
second mode of reproduction that enabled fungi to spread 
over the entire earth’s surface. At the present day the 
primitive mode of sexual reproduction has almost entirely 
disappeared in the terrestrial fungi, and its place taken by 
the conidial form of reproduction. Such is the case, for 
instance, in the large section of fungi known as the Basidio- 
mycetes, which includes the many thousands of gill-bearing 
fungi, the Polypores or bracket-fungi, etc. 
The members of the Peronosporaceae occupy an inter¬ 
mediate or transition position between aquatic and terres¬ 
trial fungi. In some species zoospores alone are present, 
in others the sexual form of reproduction gives origin to 
zoospores, while the conidial condition produces conidia 
suitable for dispersion by wind, and germinate by the 
protrusion of a germ-tube of hypha, which is capable of 
directly infecting a suitable host-plant. Phytophthora 
infestans, the cause of the much dreaded potato disease, 
is interesting in this respect, the conidia sometimes giving 
origin to zoospores, at other times germinating by a germ- 
tube or branch of mycelium. 
In most of the Peronosporaceae the sexual organs are 
