23G ECONOMIC MYCOLOGY. 



very destructive, is one of the white moulds. The myceUum of this 

 fungus is able to penetrate every part of the plant, discolouring and 

 corroding the green parts, and causing loss of vitality and decay in 

 the tuber. Partial observations of several mycologists had revealed 

 much of its life history and mode of growth during the summer, but it 

 was left for an honorary member of the Woolhope Clvib to discover 

 how it survived the winter. It has long been known that some 

 funguses, like insects, go through several stages or metamorphoses. 

 The iinal and perfect stage is easily recognised in most insects, because 

 that is the only one that has the power of reproduction ; but among 

 funguses eveiy stage is able to propagate itself in some way ; thus in 

 summer the potato blight throws off from the free ends of its mycehal 

 threads two kinds of short-lived spores, which, if they fall on the leaf 

 of a potato, germinate and quickly reproduce themselves, killing their 

 victim and perishing with it. 



Our friend Mr. Worthington Smith had the good fortune, while 

 investigating the natural history of this fungus, to discover another kind 

 of spore, called a resting spore, because it hybernates in or on the ground. 

 He watched its mode of formation in the autumn and its growth the 

 following spring, and thus was enabled to prove that this spore was the 

 long sought for means by which Peronospora infestans continues its 

 existence from year to year. 



This spore is to be found in the tissues of the decaying plant. It is 

 formed by a process of conjugation not uncommon among funguses. By 

 degrees it acquires a hard protecting coat, and, with the dying plant, falls 

 to the gi'ound, where it remains to take its chance during the winter. 

 On the return of warmth, the hard coat bursts, mycelial threads exude, 

 and extend in search of a foster mother. If they do not meet with a 

 potato plant in gi-owth, they speedily'exhaust themselves, and die ; but 

 if unfortunately successful, they pierce the cuticle, and the work of 

 destruction commences. 



Through want of thought and custom, much is done that favours the 

 existence and propagation of this pest — diseased haulm and tubers are 

 left on the surface of the ground when the crop is taken up, and are 

 afterwards dug in to serve as manure. If this happens in a garden or 

 rented potato gi-ound, and the same crop is put in a second year, a 

 vigorous crop of Penmoxpora is the result, and the cottager scarcely gets 

 his seed back. The potato blight is also extensively propagated in 

 another way. It most houses it is usual to throw away diseased tubers 

 along with parings and other rubbish into dust heaps, which are in duo 

 course cai-ted away and used as manure. It is probable that storing 

 potatos in the same buildings or floor year after year, favours the spread 

 of the disease. 



Mr. Worthington Smith's discoveiy teaches that every part of an 

 infected plant should be burnt ; it is the simplest way of effectually 

 destroying the fungus ; and also, that under no circumstances should 

 potatos be planted for two oou&ecutive years in the same ground. 



