Ce manuel présente les concepts de base de la biologie végétale, couvrant le rôle de la sève et ses fonctions dans les cellules végétales. Il fournit une compréhension fondamentale de la vie végétale, offrant un aperçu de la nutrition des plantes et de l'importance de la sève dans la physiologie des plantes.
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. Plant life and plant uses; an elementary textbook, a foundation for the study of agriculture, domestic science or college botany. Botany. CELLS AND PROTOPLASM 75 vacuole or many. (See Figure 26-) Vacuoles are spaces filled with sap. In young cells the protoplast fills the interior, but as the cells enlarge much of the interior comes to be filled with sap. Sap is the most fluid part of plants; it is simply water with various sub- stances dissolved in it. You may be familiar with the sap of maple trees whose sweetness is due to the sugar dissolved in it. Sap enters cells more freely than it passes out of them. The proto- plasm appears to encourage its en- trance and to discourage its exit. It enters living cells until there comes to be a pressure outwards, like the pressure of gas against the inner walls of a balloon. The walls of active cells are elastic and they tend to curve out as the sap presses against them. The cells press upon each other, thereby helping make the whole plant body rigid. This swollen state of the cells is called turgidity. (See Figure 2/.) The cells of a fresh leaf are turgid. The cells of a wilted leaf have lost their turgidity. Water has evaporated from them more rapidly than it was replaced. Plant cells when mature have many different forms and functions, but when young they are all much alike. They become different as they grow. This process of becoming different is called differentiation. Mature cells may be very dissimilar in appearance, as the pictures show. Yet there are certain life processes which occur in all living. Fig. 26. —A few cells from a leaf of moss, showing nuclei, chloroplasts, cytoplasm, and scattered vacuoles.. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.. Coulter, John G. (John Gaylord), b. 1876. New York, American Book Co