if the exact number and no more had been generated to form the fibre.* 

 After the vessel has reached maturity, the liquid contents themselves 

 become absorbed, as happens in the cells of the pith, and the vessel 

 is then empty ; and probably from being seen at different periods of 

 its existence in these different states, sometimes full and at others 

 empty, may account for the discrepancies existing among botanists as 

 to the functions these vessels perform. 



The period of the vessel's growth at which the laying down of fibre 

 commences, determines the distance between the several coils, for in- 

 stance, when it is first formed, the coils are quite close, scarcely any 

 perceptible trace of membrane existing between them ; if the vessel 

 have completed its development the distance is preserved, but if the 

 vessel have yet to extend its length, then as it elongates it separates 

 the coils of fibre farther from each other, and frequently, as in the pe- 

 tiole of Loasa contorta, this is carried to such an extent, that at last 

 the fibre is reduced from a spiral course around the interior of the ves- 

 sel, to an undulated line extended through it longitudinally. Those 

 vessels that are formed the earliest are always the smallest, and have 

 the coils of the fibre most separated from each other. 



In the annular vessel, the development of the cell and the adher- 

 ence of the. granules to each other are conducted in the same manner 

 as just described, the direction of the deposit showing a tendency to- 

 wards the spiral direction, by the presence of a spire connecting two 

 rings, or by a ring being developed in the middle of a spiral fibre. 



*The spiral does not always incline to the same direction, the "right-handed screw" 

 being the most general, and the " left-handed " the least frequent. Often is it to he found 

 that vessels contain more spiral fibres than one, in fact they vary from one to twenty 

 in the same helix ; and it occasionally happens, as in the spiral vessels found in the 

 petiole of the leaf of Loasa contorta, that besides the spiral fibre there are other fibres 

 extending longitudinally through the vessels, appearing as if tbe materials for the 

 formation of the spiral fibre had not been exhausted, and that these longitudinal fibres 

 (varying in number in different vessels) were tbe result of tbe appropriation of the re- 

 mainder, to form secondary deposits. The development of these secondary fibres has 

 not been witnessed, but it is imagined that they are developed from granules, as above 

 described. 



The shape of the fibre is not in all cases the same, being either semicylindrical, 

 square, flattened, or, as Meyen has described in Opuntia cylindrica, lamelliform, and 

 projecting some considerable distance into the interior of the vessel. 



A general view of the process of forming a spiral fibre may be obtained by exam- 

 ining the hairs (rootlets) on the under side of the fronds of Marchantia polymorpha 

 at different periods of their growth. 



In some rare instances forked spiral vessels are to be seen, especially in the "long 

 leek," in the tissue at the base of the bulb connecting it with the rootlets. 



