THE CAMBIUM RING. 135 



The actual nature of most of the elements constituting the 

 vascular bundle can only be realised in longitudinal sections, 

 though their distribution is best seen in the cross-section. A 

 delicate radial-long itiuiinal section, which has cut through pretty 

 near the middle of a vascular bundle, shows in that portion of 

 the bundle nearest the pith, elongated primary wood paren- 

 chyma, with horizontal end walls ; between them very narrow 

 annular vessels, or vascular tracheides, more or less crushed, 

 then somewhat broader annular vessels, perhaps partially show- 

 ing transitions to the spiral form ; then closely wound, still 

 broader, spiral vessels, partly showing transitions to the reticu- 

 lated form ; and finally the broad pitted ducts, with bordered 

 pits. Between these various forms of vessel or tracheide can be 

 seen very elongated tracheides with tapering ends, devoid of 

 contents, and also with bordered pits r and in thick-walled wood 

 parenchyma, short, with horizontal end walls, and broad un- 

 bordered pits, and usually containing starch. The immature 

 ducts appear as broad, cylindrical thin- walled cells, separated by 

 horizontal end walls, with a thick lining layer of cytoplasm, and 

 with nucleus. In older ducts the end walls are swollen, and the 

 contents reduced. In the fully-formed duct these contents are 

 no longer observable, and the end walls are reduced to a narrow 

 ridge projecting inwards as a thickening ring. The flat tabular 

 cells of the cambium ring show abundant cytoplasmic contents, 

 nuclei, and delicate partition walls. The sieve -plates of the 

 sieve-tubes are oblique, and present to the observer their surface 

 with darker shining dots. The side walls of the sieve-tubes are 

 likewise provided with small, finely-punctate sieve-pits, usually 

 elongated obliquely. In the periphery of the bast the callus 

 plates are already formed, covering the sieve-plates with 

 a highly refractive layer. The small sieve-pits on the side 

 walls also have small callus plates. By the sieve-tubes will be 

 noticed the companion-cells, densely filled with cell-contents, and 

 the broader, shorter cells of the bast parenchyma, usually with 

 starch-contents, but less protoplasm. The bast is separated from 

 the ring of sclerenchymatous elements by broader somewhat 

 more elongated parenchyma cells of the ground -tissue. The 

 sclerenchyma-fibres of the ring are very long, pointed at their 

 ends, with their ends interpolated between one another, and are 

 provided with fine pits. The elements of the starch sheath are 

 relatively short ; then follow the rounded cells of the chlorophyll- 



