METABOLIC PROCESSES IN THE PLANT. 353 



allow a vertical strip of cortex to remain as a bridge between the 

 short lower and the long upper segments of the shoot, there is a 

 comparatively luxuriant formation of roots in the lower part, 

 which is due to the fact that not inconsiderable quantities of 

 proteid can cross the bridge. 



If we examine under the microscope transverse sections of 

 willow shoots, we shall easily make out that the wood of the fibro- 

 vascular bundles on its inner side directly abuts on the pith. 

 Bast is only present between the cortex proper and the outer side 

 of the wood, and therefore the conduction of proteid must be 

 interrupted if the ringing extends right through to the wood. 



Ringing experiments with Mirabilis Jalapa or Nerium Oleander 

 yield results very different from those obtained with willow shoots, 

 and generally with shoots possessing the typical dicotyledonous 

 structure. And, in fact, the anatomical structure of the stems of 

 these plants is very peculiar. 



Transverse sections of Mirabilis stems about 4 mm. in thickness 

 show under the microscope the epidermis, the primary cortex 

 with its external ring of collenchyma, and, in particular, a ring 

 consisting of strongly lignified cells (sclerenchymatous fibres) and 

 intercalated vascular bundles, and, finally, the central part of 

 the stem. This latter consists of medullary tissue, whose cells 

 I still found to contain large quantities of starch at the end of 

 October, when the leaves of the plants had been killed by a night 

 frost, and, distributed in the ground tissue, vascular bundles 

 with distinct bast and wood. These central vascular bundles are 

 not reached at all by ringing, and consequently in 'this case the 

 removal of the ring of a cortex does not materially interrupt 

 the paths of conduction either of the non-nitrogenous or of the 

 nitrogenous plastic substances. 



We select a vigorous, very leafy shoot of Nerium Oleander, 

 remove a ring of cortex at a distance of about 20 mm. from its 

 lower end. We then pass the shoot through the cork closing 

 a vessel filled with water, fixing it in the hole of the cork in 

 such a way that it dips into the water to a depth of about 80 mm. 



If the temperature is high enough, and the air not too dry (it 

 is advisable to keep the shoots in the hothouse), many roots, after 

 some time, break out from the parts of the stem, above the ring, 

 which are in the water. After a time a fairly large number of 

 roots also develop at the base of the shoot, i.e. below the ring. 

 Xerium shoots thus behave quite differently to willow shoots, and 



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