.320 PHYSIOLOGY. BOOK II. 



tions of a grossei* 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 tlie 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 Papaveracea? he found that the 

 matter was analogous to opium, and in Leguminosae to gmn ; 

 in Graminese it consists of alkaline and earthv 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 



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