56 THIRD REPORT — 1833. 



Dr. Dutrochet is of opinion * that the whitish spots we some- 

 times see in leaves, and the paler tint that generally character- 

 izes the under side of the same organs, are owing to the presence 

 of air beneath the cuticle. He finds that the arrow-head shaped 

 blotch on the upper side of the leaf of Trifolium pratense, and 

 the whitish spots on Pulmonaria officinalis, disappear when the 

 leaves are plunged in water beneath the exhausted receiver of 

 the air-pump, and that the lower surface of leaves acquires the 

 same depth of colour as the upper under similar circumstances. 

 This he ascribes to the air naturally found in the leaves being 

 abstracted, and its place supplied with water ; a conclusion 

 which agrees with what might be inferred from the anatomical 

 structure of the parts in question. 



Excretions. — It has long been known that some plants are 

 incapable of growing, 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 succeed 

 on the site of an old apple orchard, unless some years inter- 

 vene 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 expe- 

 rience 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 CandoUe, both Humboldt and Plenck 

 suspected that some poisonous matter is secreted by roots ; 

 but it is to M. Macaire, who at the instance of the first of these 

 three botanists undertook to inquire experimentally into the 

 subject, that we owe the discovery of the suspicion above al- 

 luded to being well founded. He ascertained -j- 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 CichoracecE and Papaveracece he found that the matter was 

 analogous to opium, and in Leguminoscs to gum ; in Graminece 

 it consists of alkaline and earthy alkalies and carbonates, and 

 in Euphorhiacecs of an acrid gum-resinous substance. These 

 excretions are evidently thrown oflT by the roots on account of 

 their presence in the system being deleterious ; and it was found 

 by experiment, that plants artificially poisoned parted with the 



* Annales des Sciences, vol. xxv. p. 246. 

 t De Candolle, Physiologic Vegetate, p. 249. 



