SECOND SECTION. 



SECONDARY CHANGES. 



CHAPTER XIV. 



SECONDARY GROWTH IN THICKNESS OF NORMAL 

 'dicotyledonous stems AND ROOTS. 



I. Cambium. General Arrangement of the Secondary Thickening. 



Sect. 134. In the Dicotyledons possessing an axial bundle, in the anomalous 

 forms enumerated on p. 250, sub. 2, in the Berberidese and Ranunculacese men- 

 tioned on p. 249, and in the Peperomise (p. 249), the primary arrangement of the 

 vascular bundles and of the tissues surrounding them in the stem undergoes no 

 change after extension is complete. 



The same statement applies to a relatively small number of forms, the stem of 

 which possesses a normal ring of bundles, consisting of collateral leaf-trace bundles 

 of normal orientation, as in Saurureee, and species of Ranunculus. 



In the very great majority of Dicotyledonous stems, on the other hand, the com- 

 pletion of, the primary groups of tissue is followed" by the formation of new additional 

 elements, in consequence of which secondary changes occur in the pre-existing, 

 primary tissues (Sect. 54, p. 224). 



These changes proceed from the ring of bundles, and that is the case both in the 

 typical instances where the latter alone is present, and also in other cases where, 

 besides this, medullary and cortical bundles occur. The changes in question consist 

 chiefly in the fact, that from a meristematic zone, called the Cambium or Cambial 

 ring, which passes through the ring of bundles, new elements are added to the 

 latter in the direction of the transverse diameter of the stem, which thus receives a 

 (secondary) growth in thickness, through the addition of new elements. In short-lived 

 stems this growth may soon cease ; in long-lived ones, and especially in ' woody' 

 plants, it endures throughout life. 



As these changes go on, the number and arrangement of the primary leaf-trace 

 bundles, and of the primary medullary rays which separate them, either remain as 

 they were originally, or new bundles, the Intermediate Bundles, separated by medullary 

 rays, appear between the original ones, their appearance sometimes immediately 

 succeeding the primary differentiation of tissues, sometimes occurring later. 



Apart from the numerous modifications of detail which thus become possible, 

 the origin and position of the Cambium, and the arrangement of the elements pro- 



