R. BRAITHWxVITE ON THE HISTOLOGY OF PLANTS. 291 



gresses, there is developed in the wood part the oldest spiral 

 vessels, and at the same time the thin-walled cells of the bast 

 part ; then the central cambium loses its capacity for continued 

 formation, and the growth in thickness of the vascular bundle 

 before us is completed. 



The general arrangement of the vascular bundle in monocotyle- 

 dons is as follows : — The two ends of the radial diameter are 

 occupied by bundles of fibre cells, those of the bast being on the 

 cortical side, those of the wood next the pith, the other elements 

 becoming intermixed ; inward from the bast fibre bundle the bast 

 vessels are intermixed with bast parenchym cells, and within the 

 wood fibre bundle the tubular cells or vessels are partly surrounded 

 by, partly intermixed with, wood parenchym cells. The most dis- 

 tinct type is seen in that belonging to leaves, or going off to them, 

 as in Iris^ Phormium, Alpinia, &c. 



In the vascular bundles of grasses, the youngest wide porose 

 vessels stand at each side, so that they enclose the parenchym cells 

 of the wood part along with some narrower vessels ; internally are 

 placed two or more spiral or annular vessels in a single series. • In 

 palms we see in the centre of the vascular bundle 1-3 larger 

 vessels often with some narrower spiral vessels behind them, the 

 wood fibres being few and faintly thickened, while the bast fibres 

 are numerous and strongly thickened. In Liliacew, Tradescantia, 

 Asparagus, &c., the bast and wood- fibres are but faintly thickened. 

 In Alisma Plantago a great deviation takes place from the ordinary 

 type of arrangement. Here we find on the inner side of the 

 vascular bundle a wide air passage, which is surrounded by thin- 

 walled, elongated, unlignified parenchym cells, and then by a 

 crescent of strongly thickened wood fibres from the parenchyma of 

 the intermediate tissue. 



A peculiar condition of the two constituent parts of the vascular 

 bundle appears in roots. Here the usually elliptic bast bundle, 

 composed of internal bast vessels and external bast parenchyma, 

 stands between the radially placed rows of wood vessels in a thin 

 or thick-walled wood-like hitermediate tissue of fibre cells, which 

 gradually pass over into the tissue of the pith. 



Vascular Bundles of Dicotyledons. 

 These in their earliest stage of formation agree completely with 

 those of Monocotyledons, but in their perfect state they attain to 

 various degrees of complexity. 



