. Foundations of Botany. Botany. 146 FOUNDATIONS OF BOTANY (Sect. 106), which contains much water. It is impossible fully to explain in simple language the way in which the cells of the pulvini act, but in a general way it may be said that changes in the light to which the plant is exposed cause rather prompt changes in the amount of water in the cells in one portion or other of the pulvinus. If the cells on one side are filled fuller of water than usual, that side of the pulvinus will be expanded and make the leaf-stalk bend toward the >pposite side. The prompt- ness of these movements is
. Foundations of Botany. Botany. 146 FOUNDATIONS OF BOTANY (Sect. 106), which contains much water. It is impossible fully to explain in simple language the way in which the cells of the pulvini act, but in a general way it may be said that changes in the light to which the plant is exposed cause rather prompt changes in the amount of water in the cells in one portion or other of the pulvinus. If the cells on one side are filled fuller of water than usual, that side of the pulvinus will be expanded and make the leaf-stalk bend toward the >pposite side. The prompt- ness of these movements is no doubt in consid- erable measure due to the fact that in the pul- vini (as in many other parts of plants) the protoplasm of adjacent cells is connected. Delicate threads of protoplasm extend through the cell- walls, making the whole tissue a living web, so that any suitable stimulus or excitant which acts on one part of the organ will soon affect the whole organ. 155. Vertically Placed Leaves. — Very many leaves, like those of the iris (Fig. 44), always keep their principal sur- faces nearly vertical, thus receiving the morning and even- ing sun upon their faces, and the noonday sun (which is so intense as to injure them when received full on the. Fig. 114. — Compound Leaf of Bean with Pulvinus. (The pulvinus shows as an enlargement, in the figure about three- eighths inch long, at the base of the petiole.). 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 Bergen, Joseph Y. (Joseph Young), 1851-1917. Boston, Ginn & company
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