358 CONDUCTING SYSTEM 



in Equisctam. Traces of the disintegrated annular and spiral vessels 

 which are replaced by this intercellular passage, persist in the form of 

 characteristic thickening fibres adhering to the walls of the canal. 

 According to Westermaier, these passages contain water, at any rate at 

 certain times ; 175 it is uncertain, however, whether they merely serve 

 as reservoirs of water or whether they take an active part in the work 

 of conduction. 



3. The physiological significance of the different types of vascular hundlr. 



We must preface our remarks concerning the physiological signifi- 

 cance of the principal types of conducting strand with the statement, 

 that this subject cannot be treated satisfactorily, unless the phylogenetic 

 relations of the several forms of vascular structure are also taken into 

 account. We must, accordingly, begin with that type of conducting 

 system which may be presumed to be the oldest in the phylogenetic 

 sense. The most primitive type of vascular strand is the hadrocentric 

 bundle (concentric bundle with central hadrome), which makes its first 

 appearance in the stems of the most highly organised Mosses the 

 Polytkichaceae and recurs in the basal region of the young stem in 

 many Ferns. Both the radial and collateral types of vascular structure 

 must be regarded as derivative forms. 



In order to appreciate the physiological significance of the hadro- 

 centric type of vascular structure, we may formulate the following 

 question : Given a homogeneous non-vascular axial organ, how may a 

 central concentric vascular bundle become evolved by progressive 

 differentiation ? Let us imagine a small and simply constructed plant 

 body, consisting of an upright leafy shoot, with absorbing organs 

 attached to its lower end. The problem of conduction or translocation 

 will assuredly be of the simplest nature in the region of the stem that 

 lies below the insertion of the lowermost leaf. Here the water contain- 

 ing mineral salts in solution which is ascending to the leaves, meets the 

 synthetic products on their way downwards to the growing organs of 

 absorption ; the notion of ascending and descending sap-currents is, in 

 fact, realised in its simplest form. In these circumstances, the separa- 

 tion of the two currents is obviously most essential to the uninterrupted 

 progress of translocation. Hence the first step towards the evolution 

 of a vascular system, will be the differentiation of a definite strand of 

 water-conducting tissue, which in an upright radially symmetrical stem 

 will naturally take up a central position. This first stage in our 

 theoretical scheme of evolution is exemplified by those leafy Moss-stems 

 which possess a simple type of central strand, consisting of a water- 

 conducting bundle, surrounded by a sheath of parenchyma serving for 

 the conduction both of protein-compounds and of carbohydrates. By a 



