. A textbook of botany for colleges and universities ... Botany. 340 PHYSIOLOGY The problem, therefore, is: How can solutes pass the ectoplast usually impermeable to them ? The answer is merely in the form of a hypothe- sis, like the one already proposed to account for guttation and bleeding. If the accumulation of the solute causes a rise of turgor, it is conceivable that the very pressure itself might work such a change in the cytoplasmic membranes that they alter their permeability and permit the outrush of water and its solutes in the direction of least resistance, which will be toward the
. A textbook of botany for colleges and universities ... Botany. 340 PHYSIOLOGY The problem, therefore, is: How can solutes pass the ectoplast usually impermeable to them ? The answer is merely in the form of a hypothe- sis, like the one already proposed to account for guttation and bleeding. If the accumulation of the solute causes a rise of turgor, it is conceivable that the very pressure itself might work such a change in the cytoplasmic membranes that they alter their permeability and permit the outrush of water and its solutes in the direction of least resistance, which will be toward the free surface. Whether a renewed secretion will take place de- pends on the further activity of the cell. Given a repeated formation of the secretion, it might escape again. The hypothesis then suggests a rhythmic variation in the permeability of the cell membranes, the secretion being formed inside the Fig. 636. — Young resin gland of fir {Abies): ti, duct, an intercellular space formed by the sepa- ration of the four nucleate cells. — After Tschirch. This hypothesis is clearly inapplicable to secretions which are not miscible with water, like essential oils and resins. They are probably formed, however, in the very wall itself, and thus the material may not have to traverse the ecto- plast as resin or oil. Unfortunately, even the place of their origin is still obscure. R61e of certain secretions.—Nec- tar is gathered by many insects, some of which store it, after partial digestion, as honey. While the floral glands are being explored for nectar, the visitors become dusted with pollen and transfer this to ripe stigmas of the same or other flowers, thus insuring pollination in many cases where otherwise it might not occur (see Part III on pollination). The role of extrafloral nectar is not clear. Digestive glands, most defi- nite in insectivorous plants (p. 386), secrete enzymes (p. 399) by which the soft parl< of captured insects are dissolved. Essential oils (
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