360 CONDUCTING SYSTEM 



The radial construction of the vascular bundle in roots is also 

 advantageous in relation to the mode of origin of lateral roots. In 

 order that the continuity of the water-conducting system may be main- 

 tained, it is necessary that the vessels of the lateral roots should be 

 directly attached to those of the parent organ. It would be difficult 

 to ensure this continuity, if the newly-formed vessels had to traverse a 

 more or less extensive sheath of leptome. The connection could, in 

 fact, only be effected with the aid of a secondary meristem ; but the 

 leptome, consisting as it does mainly of sieve-tubes, is exceedingly 

 ill-fitted for producing the requisite meristematic tissue. In the radial 

 type of central cylinder, on the other hand, where the outer ends of 

 the hadrome-plates abut immediately against the pericycle, which is 

 normally the root-producing layer, the problem of linking up the two 

 sets of vessels presents no difficulty. 



How far the occurrence of radial bundles in the stems of Lycopodium 

 is capable of a physiological explanation, must be left undecided. It has 

 been suggested, that in this case the central vascular strand has been 

 produced by the phylogenetic aggregation of several previously distinct 

 strands ; if this view is accepted, the problem resolves itself into the 

 question as to the advantages consequent upon such a fusion. 



In dealing with the physiological significance of the collateral type 

 of vascular bundle, stems and leaves must be treated under separate 

 headings. As regards leaves, we may first of all consider the gradual 

 transition from concentric to collateral structure of the bundles which, 

 as the author has shown, regularly takes place in Ferns. Here the 

 hadrome-portion of the concentric strand gradually shifts further and 

 further from its original central position towards the adaxial side of the 

 bundle, with the result that a large portion of the overlying leptome is 

 pushed aside, and the entire bundle loses its concentric character. The 

 displacement often begins in the petiole, and at latest appears in the 

 principal veins of the lamina. In the smaller veins, the last remnants 

 of the adaxial leptome disappear, and the bundle becomes truly 

 collateral. 



These facts which may be quite easily verified prove that, in 

 the leaves of Ferns at any rate, the collateral structure of the vascular 

 bundle, and its characteristic orientation, are consequences of the dorsi- 

 ventral construction of the entire organ ; the same argument most 

 probably applies to the leaves of Phanerogams. It is natural further 

 to suppose, that the collateral structure of foliar bundles is in some way 

 correlated with the special physiological activities of the leaf. This 

 assumption also finds support in the data of comparative anatomy ; for 

 it is found, that the collateral or excentric tendency of the vascular 

 bundles is most pronounced, where the palisade-tissue which represents 



