. The Canadian field-naturalist. Natural history. 1914] The Ottawa Naturalist. 45 high a relative humidity, though it occurred also in cut branches, but more slowl}^ when exposed to drier room air. The abscission of internodes and of shoot-tendrils (such as those of Vitis, Ampelopsis) offers another case in point. The tendrils may either persist and serve as a permanent mechanical support for the plant, or they may be shed from the more distal portions of the new stems, as occurs at the end of the grow- ing season. The behaviour may be very well observed in ^Ampelopsis Veitchii. The internodes
. The Canadian field-naturalist. Natural history. 1914] The Ottawa Naturalist. 45 high a relative humidity, though it occurred also in cut branches, but more slowl}^ when exposed to drier room air. The abscission of internodes and of shoot-tendrils (such as those of Vitis, Ampelopsis) offers another case in point. The tendrils may either persist and serve as a permanent mechanical support for the plant, or they may be shed from the more distal portions of the new stems, as occurs at the end of the grow- ing season. The behaviour may be very well observed in ^Ampelopsis Veitchii. The internodes of the apparently chief shoots are eqtially marked in this respect. In both ordinary shoots and in tendrils, the plane of abscission lies near, but not precisely at, the base of the internode affected, and is not marked b}^ any histological dift'erentiation. In certain instances the abscission-plane is oblique, or even decurrent, such deviations being found where morphological displacement has occurred. According to the more generally accepted view, the tendril in Ampelopsis and Vitis is a chief shoot. Its norm^al position is therefor directly opposite a leaf, from the axil of which the sup- planting shoot of the second order arises. However, the tendril frequently, and even usually may, in particular individuals {Ampelopsis quinquijolia, ac- cording to m}' observation), suffer an upward displacement of as much as 20 mm., and in such event the abscission plane of the internode above the secondary shoot will be oblique in a degree commen- surate with the amount of displacement of the tendril. (Figure 1.) The fact of this morphological disturbance is of great importance in under- standing the position of the abscission-plane in the cotton pedttncle, as we shall presently see. The only other further example of shoot abscission to be here cited is that of the clumps of spines in certain cacti, of which Cereus Thurberi serves as an eXCell-. Figure l. Oblique abscission of an intemo
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