288 R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



ments of \Yliicli increase by cell division. The completed vascular 

 bundles in the various groups may be summarised as follows : — 



In JEquisetacece, the vascular bundles form a circle around the 

 large central air passage, produced by partial resorption of the pith, 

 and are separated from each other by layers of wide parenchym 

 cells, .which have thick walls, often coloured yellow or brown. 



On the side next to the pith, the wood part of the vascular 

 bundle is chiefly occupied by a large air passage, which arises by 

 resorption of a string of elongated thin-walled parenchym cells, 

 and into its cavity project, on each side, one or more annular, 

 spiral, or netted vascular cells, the circuit being completed by thin- 

 walled narrower parenchyma, which is enclosed by the above-men- 

 tioned thick-walled cells ; two other groups of smaller vascular 

 cells occur at the sides nearer the rind. 



The bast- part, composed of different elements, appears enclosed 

 by these four vascular groups, and the parenchyma surrounding the 

 air-passage on one side, the thick -walled parenchyma forming the 

 sheath of the vascular bundle on the other side. 



The wood-part consists of only two forms of cells, the annular, 

 spiral or netted tubular cells, and elongated starch-bearing paren- 

 chyma, which forms partly the circumference of the air-passage, 

 partly the connection between the two outer and inner groups of 

 vessels. In the bast-part we find three different forms of cells ; 

 the chief part consists of narrow, thin-walled, starch-bearing cells, 

 between which are 1-3 wider, more irregular cells, containing fine 

 granules or air ; externally we see an uninterrujDted series, or 

 several small groups of cells with narrow lumen and stronger 

 thickening of the walls, which are the true bast fibre-cells. In 

 longitudinal section we find the arrangement to be as follows : next 

 the air-passage are the spiral vessels, then starch-bearing bast 

 parenchyma, thirdly bast vessels or cribrose tubes, and externally 

 bast fibres. 



In Lycopodiacece. — Here the middle of the stem is occupied by 

 one or more vascular bundles, lying among intermediate tissue, and 

 these are separated from the rind by several layers of thick-walled 

 fibre-cells in Lycoj)odivm, or by spongiform tissue in Selaginella. 



The wood-part consists of vascular cells, the inner of which are 

 wider and have scalariform porose thickening, while the outer are 

 narrower, and show partly scalariform-porose, partly annular and 

 spiral thickening. The bast-part occupies all the periphery of the 



