DISEASES CAUSED BV FUNGI 57 



natural thing for a fungus to perpetuate its kind by means of 

 spores ? Certainly so, on the part of those fungi \vhich, up to the 

 present, have not learned to economize, and perpetuate themselves 

 without troubling about spores. The general progress made by 

 fungi in parasitism may be estimated by the relative extent to 

 which they have reached in being able to perpetuate themselves 

 from year to year, or in time, without the intervention of spores. 

 In other words, the more expert a fungus becomes in the art of 

 parasitism, the less it is dependent on spores for its, perpetuation, 

 and in the most perfect examples of parasitic fungi known, conidia 

 and spores alike are entirely dispensed with. 



The fungus causing the dreaded potato disease {Phyiophthora 

 infestans) illustrates this point to a certain degree. There are 

 several species of Phytophthora known, and all are destructive 

 parasites ; some, as the kind under consideration, causing serious 

 injury to cultivated crops of various kinds. Normally these fungi 

 have the two forms of reproduction, conidia or summer-spores, 

 and lesting-spores or winter-spores, which require a period of rest 

 before they germinate, which takes place during the season following 

 their production. It is these winter-spores that first infect the crop, 

 the spread of the fungus throughout the season being carried on by 

 the summer-spores, which are produced in abundance and in rapid 

 succession, are dispersed by wind, animals, etc., and are capable of 

 germination tlie moment they are mature. Now in the potato 

 Phytophthora only one form of reproduction remains, namely the 

 conidia or summer-spores, the winter-spore stage being completely 

 arrested. As previously stated, it is the function of the winter- 

 spores to set up the first infection of the host-plant each season, and 

 as this stage is missing in the potato fungus, unless some other 

 provision existed for its continuance from year to year, the fungus 

 would entirely disappear. Such provision is present under the 

 form of what has been called hibernating mycelium, that is my- 

 celium of the fungus which remains in the tubers of the potato, 

 and which grows up from the tuber into the stem and leaves when 

 an infected tuber is planted. This condition of things comes about 

 as follows. Suppose we commence with a perfectly healthy potato 

 plant, and infect the leaves with the conidia or summer-spores of the 

 potato fungus. The first indication that the conidia have ger- 

 minated and entered the tissues of a leaf is the appearance of a 

 blackish patch on both surfaces of tlie leaf. The time of its first 

 appearance depends to a considerable extent on weather conditions. 

 If the weather is dull, damp, and warm, the patch may appear 

 three or four days after infection. If the weather is bright and dry, 

 probably no infection wiU take place. Granted that the plant 

 has been infected, the blackish patches on the leaves quickly increase 

 in size and present a water-logged appearance, and around the 

 edges of the blotches a very delicate white mildew appears. This 



