384 
PHYSIOLOGY. 
BOOK II. 
half their cavity. But, besides this more subtle fluid, secre- 
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 per- 
spiration. It is, however, by the roots, that the most remark- 
able secretions are voided. 
It has long been known that some plants are incapable of 
growling, or at least of remaining in a healthy state, in soil 
in which the same species has previously been cultivated. 
For instance, a new apple orchard cannot be made to suc- 
ceed 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 enquire 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 na- 
tural orders : in Cichoraceae and Papaveraceae he found that 
the matter is 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 
