ORIGIN OF THE CONCENTRIC VASCULAR STRAND 359 



further division of labour, special channels are set apart for each of the 

 last-mentioned groups of plastic materials, the cortical parenchyma 

 being reserved for the translocation of carbohydrates, while the trans- 

 port of protein substances is assigned to a new type of conducting 

 tissue, namely, the leptome. This leptome, being a delicate tissue, and 

 hence very liable to mechanical injury, will naturally tend in an 

 inflexibly constructed organ to take up its position as close as 

 possible to the neutral axis. Since, however, the centre of the stem is 

 already occupied by the water-conducting strand, the leptome is forced to 

 assume the form of an annular sheath enveloping the hadrome. By these 

 successive steps a typical axile concentric composite bundle eventuates. 



The preceding account of the evolution of the concentric vascular 

 bundle implies, that this type of conducting strand has not been a 

 simple histological unit throughout its evolutionary history ; in other 

 words, that it has not developed by a process of gradual differentiation 

 from a homogeneous primitive conducting strand. On the contrary, 

 the concentric bundle is here assumed to have arisen under the influence 

 of the same tendency on the part of heterogeneous strands to combine 

 to form histological units of a higher order, which was previously 

 invoked in order to explain the association of fibrous strands with 

 mestome-bundles. The primitive axile conducting strand of the stem, 

 delimited from the surrounding cortical tissue by an endodermis 

 which ontogenetically, as a rule, pertains to the cortex may be regarded 

 as the first stage in the evolution of the structure known as the central 

 cylinder or stele. 175 * 



The central cylinder of the root, which consists of a radial vascular 

 bundle enclosed in a pericyclic sheath (pericambium), is homologous 

 with the primary central cylinder of the stem ; in other words, the 

 radial type of bundle is undoubtedly derived from the hadrocentric 

 type. The transition from one type of structure to the other may 

 conceivably have taken place as follows : The hadrome gradually 

 extended outwards along two or three rays, and finally broke through 

 the surrounding leptome-cylinder, which thus became divided up into a 

 corresponding number of separate portions. It need hardly be pointed I 

 out, that if the vascular structure of the root were of the concentric > 

 type, the water taken in by the absorbing region of the root-system 

 would have to traverse a sheath of protein-conducting leptome, before it 

 could reach the proper conducting channels ; consequently the extension 

 of the hadrome towards the periphery at a number of points, which is 

 characteristic of the radial type of vascular structure, not only facili- 

 tates the passage of water from the cortical parenchyma to the actual 

 water-conducting channels, but also prevents any interruption of the 

 longitudinal flow of protein- material by transverse currents of water. 



