R. BKAITHWAITE ON THE HISTOLOGY OF PLANTS. 289 



vascular bundle, and in Lycopudium runs between the separate lobes 

 of the wood-part ; its component cells are of three kinds, the fun- 

 damental mass being a strong-walled, elongated, starch-bearing 

 parenchyma, in the middle of which are bast vessels, singly or in 

 small groups, and lastly, true fibre-cells. 



In Ferns. — Here the single vascular bundle, nearly circular or 

 band-like, forms in the true stem, a circle surrounding the pith ; it 

 is usually separated from the parenchyma of the pith and rind, by 

 a sheath of parenchym cells, elongated, frequently fibre-like, 

 thickened on one or all sides. As in Lycopods, the middle of the 

 vascular bundle is occupied by the wood part, the circumference by 

 the bast part. 



The wood part consists of two, or often of three kinds of cells; 

 in the former we have elongated, pointed, tubular cells, and elon- 

 gated parenchyma ; in the latter are added true vessels with their 

 lateral walls differing in structure from their oblique transverse 

 partitions. The tubular cells forming the oldest part of the vascular 

 bundle have a narrow lumen, and annular or spiral thickening, the 

 transverse partitions having scalariform perforation. Ttie wood 

 parenchyma surrounds the tubular cells and divides the wood and 

 bast parts ; its cells are elongated, and contain starch like that of 

 higher plants ; where they touch each other they are smooth, but 

 have pores when connected with vessels. 



In the bast part we find all three forms of cells, the bulk of 

 them being thin-walled, polyhedral parenchym-cells, like those of 

 the wood part; within these are wider cells in rows, or small 

 groups, with granulose contents, or empty, and cibrose thickening 

 of the walls. These are bast vessels, and external to these, near 

 the sheath of the vascular bundle, is a group of narrow cells, with 

 yellow secondary thickening. In the rachis of many ferns, e.g., 

 Osmunda, we may observe on the inner side of the vascular bundle 

 one or more strings of peculiar parenchymatous cells, probably 

 adapted to receive secretion, and representing the resin or milk 

 vessels of higher plants. 



In Rhizocarpece. — This small group comprises only the genera 

 Marsilea, Pilularia., Scdvinia, and Azolla, and has not been much 

 investigated. In the stem of Marsilea the vascular bundle appears 

 as a ring which surrounds a central cylinder of very thick-walled 

 brown fibre cells, and is separated externally from the cortical 

 tissue by a hollow cylinder of similar cells, in series, which become 



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