020 
PHYSIOLOGY. 
BOOK II. 
tions of a grosser quality take place in plants. The honey 
dew, which is so often attributed to insects, is one instance of 
the perspiration of a viscid, saccharine substance ; the manna of 
the ash is another; and the gum ladanum that exudes from the 
Cistus ladaniferus is a third instance of this kind of perspiration. 
It is, however, by the roots that the most remarkable secretions 
are voided. 
It has long been known that some plants are inca- 
pable of growing, or at least of remaining in a healthy state, 
in soil in which the same species has previously been culti- 
vated. For instance, a new apple orchard cannot be made to 
succeed on the site of an old apple orchard, unless some years 
intervene between the destruction of the one and the planting 
of the other : in gardens, no quantity of manure will enable 
one kind of fruit-tree to flourish on a spot from which another 
tree of the same species has been recently removed ; and all 
farmers practically evince, by the rotation of their crops, their 
experience of the existence of this law. 
Exhaustion of the soil is evidently not the cause of this, for 
abundant manuring will not supersede the necessity of the 
usual rotation. The celebrated Duhamel long ago remarked 
that the Elm parts by its roots with an unctuous dark-coloured 
substance ; and, according to De Candolle, both Humboldt 
and Plenck suspected that some poisonous matter is secreted 
by roots; but it is to Macaire, who, at the instance of the first 
of these three botanists, undertook to inquire experimentally 
into the subject, that we owe the discovery that the suspi- 
cion above alluded to is well founded. He ascertained that all 
plants part with a kind of faecal matter by their roots; that the 
nature of such excretions varies with species or large natural 
orders: in Cichoraceae and Papaveraceae he found that the 
matter was analogous to opium, and in Leguminosae to gum ; 
in Gramineae it consists of alkaline and earthy alkalies and 
carbonates, and in Euphorbiaceae of an acrid gum-resinous 
substance. These excretions are evidently thrown off by the 
roots on account of their presence in the system being dele- 
terious ; it was also found, by experiment, that plants artificially 
poisoned parted with the poisonous matter by their roots. For 
instance, a plant of Mercurialis had its roots divided into two 
