FUNCTION.] ORIGIN OF SECRETIONS. 305 



instance, from the leaves, when it was produced under the 

 influence of light, air, and heat. 



The principal part of the secretions of plants is deposited 

 in some permanent station in their system ; as in the roots of 

 perennials, and the bark and heartwood of trees and shrubs. 

 It appears, however, that they have, besides this, the power of 

 getting rid of superfluous or deleterious matter in a material 

 form. In the Limnocharis Plumieri there is a large pore 

 terminating the veins of the apex of the leaf, from which 

 water is constantly distilled. The pitchers of Nepenthes, 

 which are only a particular kind of leaves, secrete water 

 enough to fill half their cavity. But, besides this more 

 subtle fluid, secretions 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 th'at exudes from the Cistus ladaniferus is a third 

 instance of this kind of perspiration. 



Some of these phenomena deserve to be more particularly 

 noticed. Dr. L. F. Gsertner has described them in Calla 

 sethiopica. He states that the vessels of the leaf are not 

 continued in this plant to the end of the awl-shaped prolonga- 

 tion at the apex of the leaf, but that the secretion takes place 

 at the extreme end of this prolongation, for the length of 

 1 to 1-5 millimetres, but is scarcely visible until the fluid 

 has collected into a drop. After the death of the point, the 

 margin of the apex of the leaf assumes the function. The 

 special organs of exudation seem to be the long pores of the 

 epidermis; as is also the absorption of the secreted fluid, 

 which is sometimes observed. Light has no perceptible 

 influence on the dropping from the leaves. Warmth alone 

 has no special action, though it has when it is combined with 

 immersion in water. The excretion was feeblest in a morning ; 

 increased towards noon ; was most copious in the afternoon 

 from two to five P.M., and declined again during the night ; 

 but this periodicity is not accurately determined. There can 

 be no doubt that the dropping arises from an excess of fluid 

 beyond that which is requisite for the nourishment of the plant. 



VOL. II. X 



