. Botanique élémentaire : théoriques et pratiques. Un texte-livre conçu principalement pour les élèves de cours de sciences liées à la science et l'art département de la commission de conseil sur l'éducation . La botanique. 146 Botanique élémentaire eau il gannot, en règle générale, obtenir une autosuffisance de la liquide nécessaire, en raison du fait que la tige de coupe dans l'air agit sur la surface coupée de manière à en réduire l'absorption de puissance. Si, toutefois, la tige est coupée sous l'eau, l'absorption a lieu comme d'habitude. Que le courant ne passe par le bois peut être montré-(i) Si un anneau d'écorce est coupé

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. Elementary botany : theoretical and practical. A text-book designed primarily for students of science classes connected with the science and art department of the committee of council on education . Botany. 146 Elementary Botany water it gannot, as a rule, obtain a sufficiency of the needed liquid, owing to the fact that cutting the stem in the air acts upon the cut surface in such a way as to diminish its absorbing power. If, however, the stem is cut under water, absorption takes place as usual. That the current does take place through the wood can be shown—(i) If a ring of bark is cut off the stem of a growing plant, as long as the wood is uninjured the leaves will not droop; and (2) if the plant is watered with some coloured solution it is found to rise in the wood, staining it. The amount of the flow of water in the wood de- pends upon the amount of transpiration. It is greatest in the summer, when the transpiration is most rapid. At this time the wood cells and vessels are full of air, hence the water must pass not through the cell cavities, but in the cell walls. Trees can be much more readily transplanted in the late autumn and winter than in the summer, because at the former times they are not so full of sap as when the leaves are fully expanded, and when they are transplanted at first the roots do not take up much water. 3. Root-pressure. — We may regard both of the move- ments of water of which we have been speaking as due to suction. There is, however, a third movement which is due to root-pressure, and which depends neither upon growth nor evaporation. If the stem of a plant is cut, and the surface is protected from evaporation, water is found to ooze out. This is caused by the pressure of the root. The amoufit can be measured by means of a, . Fig. 293. -Apparatus for measuring root- pressure.. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of the

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