ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 871 



may be continued in the xylem ; every bundle then continues to grow 

 at its lower end. The concentric vascular bundles in fleshy roots are 

 continuations of leaf-traces ; they can occur only in the unlignified 

 xylem-parenchyma of the root. 



The phloem-bundles in the xylem of fleshy roots of species of 

 (Enotliera, Gaura, and Gentiana, and of Scopolina atropoides, are 

 formed out of cambium. 



The cortical vascular bundles are also prolongations of leaf-traces 

 or of parts of them. In respect to their course through the stem, 

 they may be arranged in four classes :— (1) Cortical bundles which 

 pass in an oblique radial direction through the cortex, and gradually 

 enter the peripheral ring of vascular bundles ; (2) Those which are 

 in connection with leaf-traces of a lower node which directly enter 

 the peripheral ring; (3) Those which are in connection with the 

 cortical bundles of lower internodes in a lower node ; (4) Those 

 which have a blind ending towards the next lower node, and do not 

 unite with other bundles. 



The concentric cortical bundles have a central xylem and 

 peripheral phloem. Those medullary and cortical vascular bundles 

 which possess cambium are capable of increase in thickness up to a 

 certain point. 



Nutating Internodes.* — J. Wiesner and E. von Wettstein give 

 the following summary of observations on this subject : — 



1. Internodes of stems in the condition of undulating nutation 

 show two zones of strongest growth, one in the upper curve which 

 is directed downwards, the other in the lower weaker ascending 

 curve. 



2. In the first stage of growth, after leaving the bud-condition, 

 they are orthotropous, and have at this time a uniform growth. 



3. When dicotyledons germinate (and in many seeds even at an 

 earlier period), the originally orthotropous internodes of the seedling 

 acquire a simple curvature, passing into the condition of simple 

 nutation, the maximum of growth being, while this lasts, nearly in the 

 middle of the internode. 



4. When the simple passes into an undulating nutation, the two 

 maxima of growth are manifested ; with further growth of the inter- 

 node they approach one another, and finally coalesce at the time when 

 the undulating nutation ceases. As long as the internode is growing, 

 that maximum continues which is nearer its upper end. 



5. In an internode with undulating nutation there are therefore 

 four stages: — (1) the orthotropous condition, with cell-division and 

 very slow uniform growth ; (2) simple nutation and (3) undulating 

 nutation, with division and elongation of cells, and an irregular 

 growth with two maxima ; (4) elongation of cells, with very regular 

 growth. 



6. Those zones of an internode which grow most rapidly in length 

 contain the longest cells. 



' Anz. K. Akad, Wiss. Wien, July 5th, 1883. See Bot. Centralbl., xv. 

 (1883) p. 200. 



