transformation of those nee hel of awe ‘ie tissue 
of the plant is constructed. These constitute what are 
called secretional diseases, in which cellulose is transformed — 
into gum, resin, or manna. ‘These diseases are, properly 
speaking, only excessively vigorous phenomena of vital 
functions which in many plants are quite normal. It is re- 
markable that effusions of sap of this nature are often the 
result only of external injury, and are commonly increased 
by removing the substance excreted, as in the production 
of resin. 
The consideration of secretional diseases leads to that of 
diseases of decomposition. ‘These diseases, which are fre- 
quently confounded or blended with the former—known 
generically as. gangrene or canker—are processes of decay 
‘in which the cellulose is transformed into a muddy fluid, a 
brown humus-like powder, or a carbonaceous mass, such as 
‘never occurs ina normal state. Decompositions of this nature 
are promoted, or, perhaps, are first caused, by swarms of . 
low microscopic organisms, Fungi and Algze, which find in 
the dissolution of the organic substance a favourable sub- 
stratum for their development. It is impossible to enter in 
this place on a more minute discussion of the processes of 
fermentation, decay, and putrefaction, and of the points 
which are still in dispute as to the part played by the or- 
ganisms which always accompany these processes, and as 
to their systematic position. It is, however, certain that 
diseases of this kind are characterised by the presence of 
organisms, and are by this distinguished from secretional 
diseases. In reference to the external form in which they pre- 
sent themselves, a distinction is to be drawn between moist. 
and dry gangrene, according as the product of decomposi- 
tion of the cellulose is a fluid or a solid substance. The 
latter form very commonly first attacks the innermost layers — 
of wood, which may be, according to the colour of the sub- 
stance formed, red or white. Plants attacked by decay may 
