DISEASES GF PLANTS. |" 345 
gorous plants, therefore, though their close neigh- 
bours, will have few or no fungi on their:stems. 
The lepra increases sickness in plants, and they 
die at last of a decay, if not cleared of the fungi all 
over their cutis, and transplanted ‘n better situations 
and more proper soils. | 
, § 314. 
Gallae, or galls, are produced by a small. flying 
insect; the Cynips of Linné.  Galls are round, 
fleshy, variously shaped bodies, which are attached 
to the stem, petioles, peduncles, and the leaves. 
They are formed in the following manner: ‘The 
little insect pierces with its sting the substance of 
the plant, and deposits its eggs in the small opening 
left: The few air vessels thus injured get a dif- 
ferent direction, and twist round the egg. ‘The ir- 
ritation which the sting produces, occasions, as al- 
ways In-organized bodies, a greater flow of the sap, 
(§ 280), towards the wounded place, which is de- 
posited in greater quantity than it ought to be, and 
a ‘fleshy excrescence is the product. ‘The little larva 
which leaves the egg, is nourished by the sap, grows 
up, changes into a pupa, and escapes at last-as a 
perfect insect, which propagates itself again in the 
same way. 
It is singular, that each particular fly produces a 
gall of a peculiar form. ‘This perhaps may depend 
on the peculiar structure of the eggs of each spe- 
cies; for we find that the eggs of different insects, 
when viewed with the microscope, assume peculiar 
shapes. On the oak-treé we §nd a variety of galls, , 
likewise 
