. A manual of botany. Botany. EOOT-PRESSUEE 427 Fig. 1184. different observers; an idea of its amount may be from the fact that a medium-sized Fuchsia in a pot has been found able to send a cohmin of water up a tube of the same diameter as its stem to a height of twenty-live feet. This force is continually at work while the transmission of water is going oh, but it is not easily seen later in the year. If the stem of the vine be cut in July instead of April, no bleeding follows the wound. This is not, however, due to the absence of activity by the roots, but to the fact that a copiou
. A manual of botany. Botany. EOOT-PRESSUEE 427 Fig. 1184. different observers; an idea of its amount may be from the fact that a medium-sized Fuchsia in a pot has been found able to send a cohmin of water up a tube of the same diameter as its stem to a height of twenty-live feet. This force is continually at work while the transmission of water is going oh, but it is not easily seen later in the year. If the stem of the vine be cut in July instead of April, no bleeding follows the wound. This is not, however, due to the absence of activity by the roots, but to the fact that a copious evaporation is taking place from the leaves. In the experiment in April the conditions were different, there were no expanded leaves, and the water absorbed and sent upwards by the root consequently accumulated in the vessels of the stem, escaping at once when the latter was cut; in July the vessels had been emptied by the transpi- ration, and there was no accumulation of water there to overflow. The apparatus just described wiU show, however, that the root-pressure is stiU at work if it be used in July. The root-pressure, though always con- siderable, is not constant in amount; it is lowest in the early morning, when it begins to increase ; it continues to rise till about midday or a little later, then gradu- ally sinks. A second rise takes place towards evening, and then it sinks con- tinuously aU night. The causes of this rhythmic daily period are at present unknown ; it does not appear to depend upon variations of its surroundings, but to arise from some cause inherent in the con- stitution of the plant. Transpikation.—The modified evaporation taking place from the surfaces of the succulent parts of plants, and regulated in amount by the protoplasm of the cells, is known as tran- spiration. It is easy to demonstrate the fact of its continuous existence by enclosing a plant, or part of one, in a dry glass vessel, which can be closed so as to admit no air. Very soon the s
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbotany, bookyear1895
