VASCULAR CYLINDER OR STELE 1 19 



are usually (i) vessels or trachece, (2) tracheids, (3) fibres and 

 fibrous cells, and (4) wood-parenchyma, all of which commonly 

 have much thickened firm cell-walls consisting of lignocellu- 

 lose. The proportion is not the same in all bundles and in 

 some cases certain elements are missing altogether ; tracheas or 

 tracheids, however, are constantly present in all wood. 



The vessels or trachete {a and 0) are not cells, but long con- 

 tinuous open tubes, each formed from a row of superimposed 

 cells, many of the transverse cell-walls of which have been ab- 

 sorbed or dissolved away. In some climbing plants the cavities 

 of the vessels are 9 or 10 feet long : according to Adler's 

 measurements, the vessels of oak wood average about 40 

 inches long, those of hazel and birch about 5 inches. Their 

 walls always exhibit either annular, spiral, or reticulate thicken- 

 ing or pits. Those first formed in the bundle possess only 

 annular or spiral thickenings, and constitute the protoxy- 

 lem. 



At first all vessels contain protoplasm, but during their growth 

 the living substance is used up in the thickening of the cell -walls : 

 when fully formed they are dead empty structures which serve 

 for the conduction of water. 



Tracheids resemble vessels in the character of their cell-walls 

 and in their function : they are, however, long, single, empty 

 cells and not compound structures. 



The fibrous cells are long and pointed at both ends ; they 

 possess living contents and their cell-walls are most frequently 

 thickened and sometimes marked with small pits. Fibres (/) 

 are similar thick-walled cells which have lost their protoplasmic 

 contents and contain air or water only. 



The wood-parenchyma consists of somewhat elongated cells 

 with square, blunt ends and living contents : the cell-walls are 

 thickish and slightly pitted. In these cells starch is often stored. 



{b) Bast or phloem. — The elements composing the bast or 

 phloem are (i) sieve-tubes or bast-vessels {s) with their companion- 



