. A textbook of botany for colleges and universities ... Botany. 462 PHYSIOLOGY. 10 minutes and 48 minutes. The longer the stimulation, other things being equal, the more marked the curvature ; from which it is evident that there is an increase of the excitation with continued stimulation, and thereby the end reaction becomes more marked. Summation. — Contrariwise, it should be expected that stimulation too short to result in curvature would not be without effect. That it does produce excitation is shown by the fact that if a plant be placed alter- nately horizontal and erect, each period of s
. A textbook of botany for colleges and universities ... Botany. 462 PHYSIOLOGY. 10 minutes and 48 minutes. The longer the stimulation, other things being equal, the more marked the curvature ; from which it is evident that there is an increase of the excitation with continued stimulation, and thereby the end reaction becomes more marked. Summation. — Contrariwise, it should be expected that stimulation too short to result in curvature would not be without effect. That it does produce excitation is shown by the fact that if a plant be placed alter- nately horizontal and erect, each period of stimulation being shorter than the presentation time for that particular plant, and the interval of rest shorter than is needed for recovery, curvature will finally occur. Evidendy this is a cumulative effect; yet it is not a summation of the total successive excitations that occur during the times of horizontality, . ^ but only of the re- - — --^ ^-~-~. sidual excitation. For, if a suitable plant be placed horizontal for 30 minutes continu- ously, the reaction curvature is more pronounced than if it be so placed for ten 3-minute periods at lo-minute intervals. Clearly, while erect, the preceding excitation is slowly disappearing, and if the interval before the next stimulation is too long, recovery will be complete and no evidence of the excitation will appear in the form of curvature.^ In such experiments, therefore, it is necessary to apportion properly the intervals of rest and stimulation. Rotation. — From the above considerations it will be evident that when a plant is rotated in the horizontal plane on a clinostat, its failure to exe- cute any curvature is not at all due to a lack of excitation, for while the side a of the stem is passing through quadrant A of its rotation (fig. 694), quadrants a and c are imder stimulation almost as though for a corre- sponding time the stem were at rest. But these sides remain under stimu- lation for less than the presentation tim
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