G. MASSEE AND THE FUNGUS FORAY. 135 



The special function of the conidial form of reproduction is to 

 enable tlie fungus to extend its geographical range ; as already 

 stated, the conidia are exceedingly minute, are produced in 

 immense numbers, and in rapid succession during that period of 

 time when the host-plant is in full foliage and active growth ; are 

 readily disseminated by wind, rain, insects, birds, and other 

 agents, and are capable of germination the moment they are 

 mature ; consequently those that happen to alight on the foliage 

 of a suitable host -plant enter its tissues, and within a few days 

 form a new centre of disease from which conidia are liberated to 

 continue the extension of the species. 



As a rule parasitic fungi are only capable of growing on host- 

 plants that are allied to each other ; hence from the above account 

 it can be readily understood how rapidly it is possible for a 

 fungous epidemic to spread, more especially where numerous 

 plants of the same species are growing in close proximity, as is 

 the case with most cultivated crops. All fungous epidemics are 

 due to the rapid spread of the conidial form of the fungus. 



It is important that the significance of the conicUal form of 

 reproduction should be clearly understood, for although evolved 

 very early after fungi passed from their primordial aquatic home 

 and took possession of the dry land, and in the earliest forms to 

 be considered as subordinate to the older sexual form of reproduc- 

 tion, nevertheless the acquisition of a second mode of reproduction 

 proved to be a step in the right du'ection, and has continued to 

 become more and more highly evolved, until finally in the highest 

 and newest types of fungal evolution we find the conidial mode of 

 reproduction alone remaining, the sexual form of fruit having 

 been completely arrested. 



Returning to the Peronosporeae : after the conidial form of the 

 fungus has run its course, and the infested leaves or other portions 

 are killed by the fungus and have -fallen to the ground, the 

 mycelium in their tissues continues to grow, and gives origin to 

 numerous sexually produced reproductive bodies called oospores 

 from their mode of origin ; nlso termed resting-spores because 

 they require to remain in a r&sting or passive condition for some 

 time before they are capable of germination. 



During the winter the leaves containing resting-spores may 

 decay and completely disappear, but the oospores remain un- 

 changed ■ on the soil until the following spring, - when they 



