The physiology of the circulation in plants : in the lower animals, and in man : being a course of lectures delivered at surgeons' hall to the president, fellows, etcof the Royal college of surgeons of Edinburgh, in the summer of 1872 . within the plant are drawn in opposite directions ; countercurrents being thus established which speedily drain the tree ofits juices and cause it to exhibit symptoms of collapse—to make itdroop, in fact (Figs. 40 and 41). In this case, the presence of heated air, or the condition of dry-ness, induces opposite currents. But respiring surfaces undercertain condi
The physiology of the circulation in plants : in the lower animals, and in man : being a course of lectures delivered at surgeons' hall to the president, fellows, etcof the Royal college of surgeons of Edinburgh, in the summer of 1872 . within the plant are drawn in opposite directions ; countercurrents being thus established which speedily drain the tree ofits juices and cause it to exhibit symptoms of collapse—to make itdroop, in fact (Figs. 40 and 41). In this case, the presence of heated air, or the condition of dry-ness, induces opposite currents. But respiring surfaces undercertain conditions, say in wet or rainy seasons, may becomeabsorbing surfaces. I proved this to you by showing that a thinfluid will pass through an animal or vegetable membrane for whichit has an affinity either from above or from below, according as thethin fluid is placed above or below the thicker fluid (Figs. 42 and 43). From this it follows that evaporation, or the condition of dryness,draws or sucks the fluids out of the vessels and intervascular spaces, I 60 DE J. BELL PETTIGREW ON THE while the condition of wetness pushes or propels the fluids into we have a sucking or pulling force, and a pushing or propelling Figs. 40,. Fw. 40.—Tube (o) with expanded evaporating surface (6) covered with animal or vegetablemembrane directed downwards. The expanded portion corresponds to the roots of evaporation goes on at Z>, fluid presented at a is drawn through the tube in thedirection of the arrow c.— Original. Fig. 41.—Similar tube with expanded portion directed upward. The expanded portion inthis instance corresponds to the leaves of plants. When evaporation occurs at e, the fluidpresented at d is drawn through the tnbe in the direction of the arrow /. Evaporation isto be regarded as a pulling force.— Original. Fig. 42.—Tube (g) with expanded absorbing surface {h) covered with membrane anddirected upwards to correspond with the leaves of a plant. When a th
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