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There was one explanation, however, advanced on the first outbreak 

 of the malady in Europe, which carried conviction to my mind, that the 

 immediate or exciting cause of the disease was discovered beyond boubt, 

 and I will briefly explain the facts, which I consider conclusive on that 

 point. 



The disease was attributed by the Rev. M. J. Berkeley of England, 

 Prof. Morren of Belgium, and I think M. Payen of France, to the attack 

 of a minute parasitic fungus, Botrytis iyi/estans. Before I proceed farther 

 it may be well to explain the habits of fungi, and how they injure plants. 

 Some species flourish on decaying matter and hasten its decay. The 

 blue mould observed in stale bread affords a familiar instance. Others 

 spring from the living tissue of plants and destroy it. Of the parasitic 

 tribe, the rust and smut of wheat are well known examples. The Ruta- 

 baga, if grown on a dry soil in this climate, appears liable to mildew. The 

 lower leaves may be seen to shrivel prematurely and die, and this, as I 

 have observed, may take place repeatedly, so that the leaves instead of 

 springing nearly direct from the crown of the root, are elevated on a 

 short stalk, and these roots are not unfrequently hollow and decaying. 



The way fungi effect the destruction of plants is this : the substance of 

 a plant is composed of minute cells and vessels — cellular and vascular 

 tissue. Some contain air, some convey the crude sap, and others are de- 

 positories in which the elaborated products of the plant are stored up. 

 While the plant is living and in health the varied contents of the tissue 

 are under the control of vitality, but when attacked by a fungus the cells 

 are ruptured by its mycelium or spawn, the contents are mixed together, 

 released from the controlling power of vitality, and become subject to the 

 laws of chemical affinity ; hence, putrefaction and death of the part at- 

 tacked ensues. Most will have observed white threads or filaments run- 

 ning through decaying horse manure, this is the plant of some fungus, 

 suppose a mushroom, the mushroom which rises to the surface is the 

 fructification, the reproductive part of the plant which contains bodies 

 analgous to the seeds of plants of higher organism. When first the po- 

 tatoe plants are attacked by the parasite, small brown blotches may be 

 noticed on the leaves, generally towards the edge ; if these are carefully 

 observed they will be found to increase gradually in size, and on a moist 

 dewy morning, or in damp weather, there may be perceived just outside 

 the margin of the blotch on the underside of the leaf, a greyish appear- 

 ance, this is caused by the fructification of the parasite. It commences 



