INTRODUCTION. 
XXIX 
a conidium, or spore-body, such as will be produced, by scores, 
on every fertile thread of the mould when mature. This 
conidium is an elliptical, colourless, minute body, having a 
thin outer coating of membrane, with fluid contents. These 
contents soon become granular, and at length collect at three 
°r four centres, which condense and ultimately aro distinctly 
■separated from each other by the growth of a special envelope. 
Ultimately the membrane of the mother cell is ruptured, and 
ihe three or four smaller bodies which have been differentiated 
m its interior escape, each one furnished at one extremity 
With a pair of delicate moveable hairs, by means of which 
these little bodies, now termed zoospores, can swim actively 
'n any thin film of moisture upon which they may fall. 
Possibly this film may bo on the leaf of a foster plant. In a 
short time all motion ceases and the zoospores come to rest, 
the pair of delicate cilia are absorbed and a germinating thread 
ls 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, ancl a little mass of 
threads, called a mycelium, is soon developed within the tissues, 
capable of spreading itself through (lie 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 
hranclilets, just like the original conidium whence the 
Zoospores were developed. When fully matured each fertile 
thread produces a score or moro of these conidia, which fall 
away when ripe, and then undergo transformation 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 conidia may be transported to a considerable distance 
hy a breath of wind it is impossible to say, but it is known 
that they are capable of suspension in the air, and that they 
may be carried to any spot where there is sufficient moisture 
i°r the conidia to bo differentiated into zoospores, and after- 
wards come to rest and germinate. This process takes place in 
jammer and autumn, but there is yet another means by which 
f he pest is disseminated in the spring. 
The mycelium which flourishes within the substance of the 
plant infested is capable of producing larger globose bodies, 
* hiefly within the stems, concealed from external view. These 
globose bodies secrete a thick envelope, mostly of brownish 
°°lour, and after development they remain in a state of rest 
Within the stems during the winter. So that old stems of 
Plants which are infested with the mould during the autumn 
