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Книга по требованию: Hydration and Growth (Classic Reprint)

Товар № 11247065
Вес: 0.31 кг.
Год издания: 2015
Страниц: 188 Переплет: мягкая обложка
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Excerpt from Hydration and Growth

Three main conceptions concerning growth and its developmental aspects in plants are to be met in the history of physiology in the half century beginning with the researches of Sachs and his school. The first or earliest, that of special stuffs or substances necessary for the initiation of growth and differentiation of various organs, especially for the origination and development of reproductive organs, is now giving way to the modern conclusion that "formative" material as such has no actual existence in fact and no good basis in theory. The present trend of thought leads to the assumption that growth proceeds from and depends upon states or combinations of material or accumulations in connection with living matter rather than upon any special constructive stuff or substance. This may also be held to apply to hormones, vitamines, and other symbolic expressions for combinations of material necessary for initiating and maintaining development, reproduction, or growth.

The second aspect of the subject is that which deals with the incorporation of new material in the cell and its subsequent distention by an osmotic mechanism, upon the basis of researches of Pfeffer and de Vries. The protoplast is dealt with as a sac. The products of the metabolic processes converge in the vacuole, which in consequence becomes the seat of osmotic forces and the center of the mechanism of distention. An important feature of this scheme of operation is an ideal "semi-permeable" membrane, not morphologically identifiable, internal to the cellulose wall or other durable and visible integument. The exploitation of the theory of permeability of this membrane has been carried out in such manner as to place undue emphasis on the action of the external layer of the protoplasm. The basic conception of the diffusion of material into the vacuole remains sound, and the differentiating action of protoplasm by which the increase in the contents of the vacuole sets up an internal pressure expressed as turgidity is undeniable. But the attempt to base all features of water-relations, turgidity, and growth upon the action of solutions has been proved inadequate and has resulted in an obvious neglect of the play of molecular forces in surface tensions, in imbibition, and other activities of matter in a colloidal state.

The third group of inquiries has been directed toward measurement for the purpose of establishing the physical constants of growth. Auxesis or developmental enlargement in living things has been mistakenly dealt with as a unified process, or as a series of successive reactions in studies of temperature effects, by many writers, and coefficients of some apparent validity within a small part of the range within which growth takes place have been found. Growth is a constellation of activities and the rate of one of these dependent on temperature may be the determining one when the particular process forms either the retarding or leading agency.

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