376 



CONDUCTING SYSTEM 



blindly in the cortical parenchyma. In the latter event the leaf-traces 

 can only obtain their supply of water from the central strand indirectly, 

 through the mediation of the intervening parenchymatous cells. The 

 imperfect organisation which is exhibited by the conducting system, in 

 such cases, is of special interest, because it proves that the vascular 

 bundles of the leaf are not necessarily homologous with those of the 

 stem ; hence it is quite possible, that the prevailing continuity of the 



Pig. 149. 



T.S. through the vascular bundle of the stem of Najas major. The centre of the 

 strand is occupied by an inter-cellular passage surrounded by a circle of conducting 

 parenchyma cells, which in turn are enclosed in a sheath of leptome tissue ; the entire 

 bundle is enveloped in the endodermis. 



vascular system in Higher Plants has been brought about by the fusion 

 of phylogenetically distinct foliar and cauline strands. 



In the genus Lycopoclium, and in a number of aquatic Phanerogams 

 (such as Bulliarclia aqnatica, Potamogeton, Hippuris, Myriophyllum, 

 Ceratophyllum, Najas, Eloclea canadensis, etc.), the axile strand almost 

 certainly owes its origin to the [phylogenetic] fusion of several vascular 

 bundles. As far as the Phanerogams are concerned, such a fusion 

 would merely represent a reversion to the primitive state from the 

 derivative decentralised condition. The internodes of different species 

 of Potamogeton illustrate various stages in this process of fusion. In 

 Potamogeton natans and P. perfoliatvs, the several bundles of which the 

 axile strand is composed are still clearly distinguishable. They are, 

 however, already very closely associated with one another, being 

 separated only by a few layers of parenchymatous cells, which repre- 

 sent the remains of the pith and primary medullary rays. Small 



