64 THE TISSUES OP PLANTS 



is transmitted. In some instances it is deposited concen- 

 trically, as for example, in the fibrous portion of tlie wood, 

 and it goes on accumulating until the tubular character of 

 the fibre cells is wholly obliterated. 



In the difierent varieties of vaaiform tissue and ducts, the 

 tubular character of ' the tissue permanently remains. 

 These vessels originate from the union or confluence of 

 porous, rayed, annular, and spiral cells. 



There appears to be a tendency in the secondary deposit 

 to arrange itself spirally on the inner parietes of the duot- 

 cells. In some instances the spiral is perfectly formed, 

 its several coils lying close together in the vessel and ad- 

 hering firmly to its walls. When, therefore, the vessel is 

 ruptured, its membrane tears in the direction of the turns 

 of the spiral, with which it is ultimately united and from 

 which it cannot be distinguished ; but when the turns of 

 the spiral are more or less separated from each other by 

 the elongation of the vessel, its walls are apparent between 

 the several coils, and the existence of the spiral fibre within 

 its cavity is at once demonstrated. 



Whilst the coils of the spiral lie close together in the 

 cell, they are very apt to anastomose. Sometimes several 

 of the fibres will unite throughout their entire extent and 

 produce a banded spiral. This disposition is frequent in 

 monocotyledonous plants, particularly in the banana. Fre- 

 quently the anastomosis between two or more turns of the 

 fibre is only partial, and in this way the so-called reticulated 

 spiral fibre is probably produced. In some cases^ as the 

 cell elongates, the fibre breais up into a series of rings, 

 and annular vessels are the result. This form of the spiral 

 fibre is common in monocotyledons. In place of rings, the 

 fibre may separate at regular intervals and form bars, or 

 may be even so broken up as to appear in the form of dots 



