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The Life of the Plant. 285 
still live a long time under certain circumstances, and may 
have a perfectly sound appearance on the outside, until 
the disease reaches the cambium-layer, when they quickly 
perish. ‘The decomposition of cellulose does not, however, 
always begin inside the plant; it may also penetrate from 
without, at spots which have become diseased from ex- 
ternal injury. | 
Diseases of another class, produced by vegetable parasttes, 
that is by plants which derive their nourishment from the 
living tissue of the plants they inhabit, are in general less 
obscure than those which were last described. » 
Among these parasites are certain Fungi (Fig. 374), 
which are of the greatest practical importance. The ques- 
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Fic. 374.—Destruction of the cell-wall caused by a Fungus: transverse section of the 
wood of a beech ; at a, a the destruction of the secondary layers is still in pro- 
gress ; at 6, 6 the entire cell-wall has been destroyed with the exception of a 
larger or smaller residue of the primary layer, and the cell-cavity filled with 
an amorphous substance. 
tion which has often and for a long time been discussed _re- 
specting these plants, whether they are the cause or only the 
accompaniment of the disease, has now been decided in 
favour of the former alternative, since it has been proved 
that the organisms in question are well-defined species of 
Fungus, and that the same organism is always found to ac- 
company the same disease. In addition to this, it has been 
Q 
