. A Manual of botany : being an introduction to the study of the structure, physiology, and classification of plants . Botany. CIRCULATION OF THE SAP. 147 through the woody tissue, porous vessels, and cells, dissolving starch and other matters, and appropriating various new substances. Pro- ceeding upwards and outwards, this sap reaches the leaves, where it is exposed to the air, and is elaborated by the function of respiration. It then returns, or descends chiefly through the bark, either directly or in a circuitous manner, communicating with the central parts by the medullary rays, depositin
. A Manual of botany : being an introduction to the study of the structure, physiology, and classification of plants . Botany. CIRCULATION OF THE SAP. 147 through the woody tissue, porous vessels, and cells, dissolving starch and other matters, and appropriating various new substances. Pro- ceeding upwards and outwards, this sap reaches the leaves, where it is exposed to the air, and is elaborated by the function of respiration. It then returns, or descends chiefly through the bark, either directly or in a circuitous manner, communicating with the central parts by the medullary rays, depositing various secretions, more especially in the bark, and giving origin to substances which are destined to nourish and form new tissues. Finally, it reaches the extremity of the root, where absorption commenced; a small portion is there excreted, while the remainder mixes with the newly-absorbed fluids, and again circulates in the sap. The rapidity with which the sap ascends is dependent on the endosmotic property of the cells in the roots, and on the density of the fluids. An absorption of water, con- taining various matters in solution, is constantly going on through the extremities of the rootlets. The sap thus formed is carried forward through the cells, vessels, and intercellular passages, by a force which acts by propulsion. The stimulus of light, acting on the cellular tissue of the leaves, enables them to elaborate the organic compounds which are necessary for vegetable nutrition. The leaf-action may be reckoned one of attraction or suction, > transpiration giving rise to a constant flow of fluids to supply the place of those exhaled. Dr. Pettigrew has given the following views as to the circulation in plants, and has illustrated them in the accompanying diagram (fig. 242). In spring the sap being mainly concerned in the growth of the branches, development of buds, and evolution of leaves—a vigorous and rapid movement takes place in an upward direction, as at a. Duri
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectbotany, bookyear1875
