40 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. 



Preparation now moved near window with southern exposure, as a eon- 

 sequence of which it was warmed by the direct rays of the sun. 



Two additional measurements gave the following results, showing that 

 the total length of the shoot decreased from 67.4 in the earlier part of 

 October to 61.5 in November, a shrinkage of about 7 per cent. 



Some of the peculiarities of the nutating movements of etiolated stems 

 have been described by Wiesner, who saw more than one zone of curva- 

 ture in one internode at the same time. (See Wiesner, J . , Die undulirende 

 Nutation d. Internodien, Sitzungsber. d. Wien. Akad., lxxvii, Abth. 1, 

 1873, Bewegungsvermogen, p. 22, 1881, and Siszungsber. d. Wien. Akad., 

 lxxxviii, Abth. 1, p. 454, 1883.) 



The etiolated shoots of Opuntia consisted of many internodes, all of 

 which were more than a year old. The transverse section showed a flat- 

 tened oval with a central mass of parenchymatous tissue, comprising about 

 one-third of the shortest diameter. Inclosing this is a flattened oval of 

 15 to 20 fibro-vascular bundles, with perhaps a few secondary formations, 

 and external to these was a heavy layer of parenchymatous tissue, which 

 in thickness amounted to about one-third of the diameter and had the posi- 

 tion and some of the features of a cortex. The epidermis was simple and 

 with walls not materially thickened. (See fig. 1.) 



Shoots taken from the dark-chamber and exposed t< > open-air conditions 

 until they became green, displayed a reduction of the turgidity of this 

 "cortical layer," so that the walls were variously contracted and folded, 

 the central parenchyma being only slightly affected. Greened shoots used 

 as xeno-parasites shoved a marked accentuation of this collapse of the 

 external parenchyma, which culminated in the wrinkling and death of the 

 epidermis and of the outer layer suggestive of decortication. This process 

 proceeded slowly for several months, and its course is similar to some of 

 the changes which take place in the basal portions of the stems of many of 

 the opuntias. 



"All active nutation curvatures are the result of unequal growth on the 

 two sides of the cell or curving organ" (Pfeffer, Physiology of Plants, in, 

 p. 12, 1906), but the alternating curvatures of Opuntia under discussion 

 are seen to be due to causes exactly the reverse of those to which nuta- 

 tory phenomena are ordinarily attributed. A second characteristic con- 

 sists of the fact that the entire shoot displays this movement for several 

 months; and the third characteristic, equally notable, is the alternation of 

 curvatures by which the convexity or concavity passes down one side of the 

 shoot to the base and then passes to the opposite side, in which it moves 



