. A compendium of general botany. Plants. TISSUES AND SIMPLE ORGANS. 101 downward along the cortex; wherever the path is broken, this substance is at once converted into callous tissue. The movement of undissolved albuminous substances in the sieve-tubes, like that of the milk-sap mentioned above, is a mass- movement. The causes for this movement, though not definitely determined, have already been referred to. Gravity, outer me- clianical pressure upon the soft elements, and turgor-oscillations in the neighboring tissues no doubt assist in bringing about this movement. (i. Conduction of Carbo
. A compendium of general botany. Plants. TISSUES AND SIMPLE ORGANS. 101 downward along the cortex; wherever the path is broken, this substance is at once converted into callous tissue. The movement of undissolved albuminous substances in the sieve-tubes, like that of the milk-sap mentioned above, is a mass- movement. The causes for this movement, though not definitely determined, have already been referred to. Gravity, outer me- clianical pressure upon the soft elements, and turgor-oscillations in the neighboring tissues no doubt assist in bringing about this movement. (i. Conduction of Carbohydrates. Here, and in general with substances in solution which must pass through plasmic membranes and cell-membranes, we are con- cerned with molecular Tnovements., which belong to the domain of osmosis. General statements only will be made now; particulars will be given below. The physical considerations of osmotic action differentiate (1) hydro-diffusion.^ the osmotic interchange of two miscible substances without any separating membrane; from (2) diosmosis in the usual sense, tliat is, a process similar to (1) with a dyalizing membrane or porous substance. Both processes occur in the vegetable cell. Ad 1. In an assimilating palisade-cell exposed to the sunlight several currents must be formed in obedience to the principle that the current is fonned at right angles to the lines of equal concentration (see Fig. 60). It is assumed that the maximum concentration of sugar, for ex- ample, is at 5 (Fig. 60). The sugar-molecules will then move in the direction of the arrows; the water- molecules in the opposite direction. Wherever a crystal or a starch-grain grows within a cell, there are produced such zones of concentration in the surrounding liquid, and the respective movements mil take place. Ad 2. If the solutions in neighboring cells are of Haberiandt.) unequal concentration, a new complication arises, which leads us into a branch of physiology, in part, yet unexplained. The
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