xiii] CONDENSED VASCULAR CYLINDER 175 



The central strand, even when extremely simple as in the case 

 of Callitriche^^ the Hydrilleae, Aldrovandia^ Naias^ Hippuris^, 

 etc., is not a single bundle, but represents an entire vascular 

 system, in which the strands are not differentiated as indi- 

 viduals. That the xylem reduction, to which we have already 

 referred, is not itself the cause of the union of the single bundles 

 into an axial strand, may be deduced from a comparison with 

 the stems of colourless saprophytes or parasites. In such plants 

 there is little transpiration and no assimilation and the xylem 

 is proportionately reduced. But the simplified bundles retain 

 their ancestral position and do not fuse into an axial strand^. 



Among the Dicotyledons there are certain hydrophytes, e.g. 

 the Water Buttercups (Fig. 1 13, p. 176), in which the bundles 

 remain perfectly separate, but in the majority some degree of 

 condensation may be observed. The Potamogetons (Fig. 39, 

 p. 62 and Fig. 40, p. 64) provide an exceptionally interesting 

 series illustrating, within a single Monocotyledonous genus, 

 stages in the concentration of the vascular cylinder. It must 

 suffice here to draw attention to a few other typical examples, 

 showing various grades in the reduction of the vascular system. 



In Peplis Portula there is a well-marked axial strand, in 

 which individual bundles can no longer be distinguished. In 

 transverse section, an external ring of disconnected phloem 

 groups is seen to enclose a ring of xylem, consisting of short 

 radial rows of vessels separated by rows of parenchyma. The 

 internal phloem characteristic of the Lythraceae is developed 

 within the xylem, and a pith is formed. A cambial layer occurs, 

 but does little work. 



The next stage of reduction may be illustrated by the stem 

 of Callitriche (Fig. 114 y^and 5, p. 176) which shows in trans- 

 verse section a small ring of xylem surrounded by phloem; 

 there is no cambium. In the water forms (Fig. 1 14 5) the pith 

 is resorbed at an early stage and is represented by a space. 



Hippuris has travelled still further upon the road of speciali- 



1 Hegelmaier, F. (1864). 2 Sanio, C. (1865). 



3Schenck, H. (1886). 



