362 CONDUCTING SYSTEM 



the derivation of a central cylinder composed of several collateral bundles 

 from the primitive type of central cylinder which consists essentially of 

 a single hadrocentric strand. Now it is by no means unusual, even in 

 the case of roots, for the middle of the central cylinder to be taken up 

 by a parenchymatous pith, so that the vascular tract is annular in 

 transverse section. Similarly, the middle of the primitive hadrocentric 

 bundle of the stem has in the course of evolution been replaced by a 

 core of medullary parenchyma of gradually increasing diameter ; the 

 vascular tract of the central cylinder has accordingly expanded into a 

 double ring or rather a hollow cylinder composed of hadrome and 

 leptome. i75b 



It is impossible to decide, whether medullation, or the acquisition 

 of centrifugal tendency by the vascular tissue, was the primary pheno- 

 menon ; in all probability both factors have contributed to the final 

 result. Before the differentiation of a mechanical system which, as 

 already stated, is phylogenetically a comparatively recent development 

 the duty of strengthening the various organs rested chiefly with the 

 thick-walled hadrome, a state of things which still persists in many 

 weakly constructed plants. In these circumstances the hadrome tended 

 to shift towards the periphery, in organs which had to resist bending 

 strains, and at the same time to assume the form of a hollow cylinder ; 

 as a secondary consequence of this centrifugal displacement of the 

 hadrome, the leptome was also forced further outwards. A demand for 

 an improved water-supply to the transpiring cortex may have helped to 

 " attract " the hadrome towards the surface, in the stem as well as in 

 the leaf. On the other hand, the formation of a core of medullary 

 parenchyma also entails a number of special advantages, of which one 

 deserves particular mention. By the development of a pith, the stem 

 acquired a tissue which could be employed for the temporary storage of 

 various substances, with the result that the primary cortex could be 

 more effectively utilised for purposes of translocation. 



The breaking up of the primitive continuous leptome-hadrome 

 cylinder into a circle of distinct collateral bundles was probably a 

 secondary change, brought about by the necessity of ensuring free com- 

 munication between the conducting tissue of the cortex and the storage- 

 cells of the pith. This necessity led to the formation of the primary 

 medullary rays, which, to begin with, no doubt merely represented 

 centrifugal extensions of the medullary parenchyma, that broke up the 

 hadrome, without interrupting the continuity of the leptome-cylinder. 

 The careful researches of Zanetti have shown, that this condition persists 

 at the present day in the stem of Osmnnda regalis. In this respect, 

 therefore, Osmunda may be regarded as a relic of that stage in the 

 evolution of vascular plants, at which the primitive continuous con- 



