828 



insects, and which do not increase after the marauders have made 

 their meal. The first obvious indication of the disease is a withered, 

 bhiish spot, brown in the centre, where the leaf is killed, and rapidly 

 extending, and which always originates on the under surface of the 

 leaf. Closer observation discovers a white halo, or mould, around 

 the spot, and this is a certain indication of the total destruction of 

 that portion of the plant above the ground, and also of every tuber 

 which is subjected under favourable circumstances, as moisture, to the 

 infection. I shall by and by prove that this mould is the identical 

 cause of the destruction of the tuber. Under a lens of very moderate 

 magnifying power it has a cottony aspect, and is diffused among the 

 hairs of the leaf, cutting them up and disarranging them, and from it 

 arise very numerous thready, forked stems, having fruit-bearing, oval 

 capsules, which appear to me on a rough calculation to have a 

 long diameter equal to y^W of an inch, and which, when ripe, shed 

 their seed or sporules, which become, no doubt in myriads, the pro- 

 perty of the atmosphere. But the cottony root or spawn, when once 

 produced, appears capable of multiplying itself indefinitely, at least 

 until it has effected the destruction of that on which it feeds. It ex- 

 tends to the stalk upon which the fruit capsules are, under favourable 

 circumstances, most clearly demonstrated. 



" Leaves' are the lungs or resjjiratory organs of plants. Soon 

 therefore after the appearance of the leaf-spot, stains appear on the 

 stalks, and are the result of impeded, and then checks, circulation of 

 the sap. In a few days from the first infection, all that portion of the 

 plant above the ground is, as it were by one fell swoop, destroyed. 

 The evaporation of its aqueous vapour is speedily accomplished, and 

 a little carbonaceous and saline matter in the shape of a dead stalk 

 indicates the situation of the tubers, which are found more or less 

 loosely connected with it by decaying attachments. 



" 2. The Disease of the Tuber. 



" On digging the tubers it very commonly is observed that one 

 only of a bunch is the subject of the disease, which it has derived 

 either from an infected parent set (or portion first planted), or from 

 the leaves and stalk, and is, I believe, always in the closest approxi- 

 mation to them. 



" Much moisture on the tuber renders its surface unfit for micro- 

 scopic examination, and upon the dry surface of the tuber the fungus 

 may not luxuriate, although it has first passed through it on its way 



