CHAPTER XV. 



SECONDARY CHANGES OUTSIDE THE 

 ZONE OF THICKENING. 



Sect. i68. In the normal Dicotyledons and Gymnosperms the mass of wood 

 and bast developed from the cambium is bounded on the one hand by the pith, and 

 the pre-existing ligneous zones adjacent to it, on the other by the external cortex 

 and the pre-existing bast. It is clear, a priori, that this surrounding tissue may, and 

 to some extent must, undergo changes, in consequence of the cambiogenetic 

 secondary growth. 



As stated above, no change in the pith is necessarily involved in the actual 

 course of the processes of secondary growth described, or as a consequence of 

 them. As a matter of fact, however, it has been asserted, especially by Duhamel ', 

 that in trees and shrubs the medullary cylinder diminishes in thickness, and 

 may at last wholly disappear as the secondary growth of the wood proceeds. As 

 regards the majority of cases, this view has been given up, as depending upon 

 imperfect observation, and in fact the only demonstrable anatomical change in 

 the pith during the phenomena of secondary growth is, that it sooner or later, 

 rapidly or slowly, dies oflf and dries up. The possibility of a change in the 

 pith caused directly by the growth in thickness is not indeed excluded a priori. For 

 if wood and bast increase in thickness and circumference, and the external cortex 

 does not yield to this enlargement of the circumference in a corresponding degree, 

 arr increasing pressure will be exercised on the pith, which may lead to anatomical 

 changes in the latter. In what cases and in what form such changes may possibly 

 take place, are questions which have not been investigated, and to the solution of 

 which there is scarcely any safe clue ; the possibilities will not be discussed here. 

 That such cases occur is however shown by the change of form in the pith of the 

 internodes of Aristolochia Sipho, which accompanies the growth in thickness. The 

 young internode, up to the age of one year, has an approximately circular transverse 

 section. The pith, which is of the same form, or is broadly elliptical, as seen in 

 transverse section, consists, like the medullary rays, of permanently delicate and soft- 

 walled cells, and is surrounded by a circle of n-13 leaf- trace bundles", without 

 intermediate strands. The external cortex enclosing the latter contains a strong 



' Physique des Arbres, I. p. 37; detailed discussion in De CandoUe, Organographie, I, p. 168, 

 ' Compare Nageli, Beitrage, p. 82, Taf. VIII. 



