Page:An introduction to physiological and systematical botany (1st edition).djvu/100

70 confined to the bark, but deposited throughout the substance or wood of the root, as in Rhubarb, ''Rheum palmatum, Linn. fil. Fasc. t. 4, and Gentian, Gentiana lutea and purpurea, Ger. emac. 432, f.'' 1, 2. In the wood of the Fir indeed copious depositions of turpentine are made, and in that of every tree more or less of a gummy, resinous or saccharine matter is found. Such must be formed by branches of those returning vessels that deposit the new alburnum. These juices appear to be matured, or brought to greater perfection, in layers of wood or bark that have no longer any principal share in the circulation of the sap.

The most distinct secretions of vegetables require to be enumerated under several different heads.

Gum or mucilage, a viscid substance of little flavour or smell, soluble in water, is very general. When superabundant it exudes from many trees in the form of large drops or lumps, as in Plum, Cherry, and Peach-trees, and different species of Mimosa or Sensitive plants, one of which yields the Gum Arabic, others the Gum Senegal, &c.