1914] 



The Ottawa Naturalist. 



45 



high a relative humidity, though it occurred, also in cut branches, 

 but more slowly, 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 verv well observed in 

 Ampelopsis Veitchii. The internodes of the. apparently chief 

 shoots are equally 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 

 by any histological differentiation. In certain instances the 

 abscission-plane is obHque, 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 normal 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 quinquifolia, ac- 

 cording to my observation), 

 suft'er an upward displacement 

 of as much as 20 mm., and 

 in such, event the abscission 

 plane of the internode above 

 the secondarv 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 

 peduncle, 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 C e r e us 



Thurberi serves as an excell- . Figure l. ObUque abscission of an intemode 

 1 r^y^ n 1 r • '^ conseqiitnce ot the upward displacement of 

 ent example, i he neshV fruits the tendril which normally occurs directlv 

 of fhic cr>Poipc arp cn\Tiprf^c\ h\T ?PP°site a leaf. The normal relation is shown 

 OI tniS species are COVerea Oy ,n the upper figure. (Partbenium argentatum.y 



