NUTRITION IN VEGETABLES. 71 
Vegetable substances, though more tardily, ultimately yield 
; to the same agencies. 
249, It has been observed that many plants excrete from 
their roots a peculiar matter which varies in the different 
kinds of plants. This matter, it is probable, consists of 
those parts of the sap which are not adapted for the nourish- 
ment of the vegetable, and which have been absorbed along 
_ with the nutritious particles, as the spongioles, it is supposed, 
: have not the power of distinguishing between the different 
substances that are presented to them, but absorb promis- 
_ euously all fluids with which they come into contact. 
250. This matter, being rejected by the vegetable, must 
therefore be injurious, or at least of no use, to a vegetable 
of the same kind; and hence a succession of crops of the 
same plant, or even of the same family of plants, become 
gradually degenerated and of an inferior quality ; while, on 
the other hand, any crop or succession of crops is followed 
with advantage by a crop of another family, as it is found _ 
that it can make a good use of the exudation from the roots 
of the former. 
251. This excretion from the roots may be considered a 
kind of manure, calculated, however, to be beneficial only to 
plants of a family different from that which produced it; 
and this is a principle capable of a most important and ex- 
_ tensive application in agriculture, as, by a judicious succes- 
sion of crops, we are enabled to provide from each rope 
sort of manure for that which is to succeed it. It is in this a 
manner supposed that a crop of leguminous plants prepares __ 
the ground for the succession of Corn, Barley, Oats, and 
the like. And it is also on the same principle that we: 
certain plants affect others in their vicinity, as the 
salicaria, the Willow. 
252. The fluids thus absorbed undergo some 
in their course from the fibrils of the root to 
ing most probably acted upon by the 
