R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 287 



civm, and some Hypna, tlie centre of the stem is occupied by a 

 bundle of elongated parenchym cells, by wliicli the sap is conveyed 

 upward. In Mnm?n the central bundle is more distinct, and con- 

 sists of thin-walled polygonal cells, enclosed by a ring of thickish- 

 walled yellow cells. In Polytrichum a transverse section of the 

 stem shows us that the centre is occupied by a cylinder of thickened 

 reddish cells, around which is another of thin-walled cells, having 

 intercellular spaces ; external to this is one of regular thin-walled 

 yellowish cells, and lastly another, one or two rows wide, with thin 

 dark-brown walls, which forms the sheath of the vascular bundle. 

 By longitudinal section we see that the central cylinder is a bundle 

 of greatly elongated cells, with horizontal or oblique transverse 

 partitions, and some fibre -like cells, surrounded by another of thin- 

 walled parenchyma ; next comes the woody. cylinder, consisting of 

 still more elongated cells, with albuminous contents, and by this 

 part of the vascular bundle it appears that the sap is conducted, 

 while the inner must be regarded as corresponding to pith. Looked 

 at from a structural point of view, we must place the species of 

 Polytrichum at the head of all the mosses, since in them the tissue 

 elements attain the greatest amount of differentiation. 



Vascular Bundles of the Higher Cryptogams. 



These consist of tubular and elongated parenchym cells, with but 

 a small amount of fibre cells, and in all the full development of 

 the xjonstituent parts is rapidly attained. This may be best followed 

 out in Lycopods and Ferns ; in the Lycopodiace(^, the elements 

 separate from the cambium cylinder at different points, and we 

 observe internally certain narrow, quickly lignified, vascular cells, 

 with annular or spiral thickening, and externally narrow non-ligni- 

 fied elements of the bast part, and from these new wood and bast 

 elements develope. 



In ferns the first formation commences at one or two points ; in 

 .the cambium string there first separate one or two fine spiral vessels 

 near the periphery, and opposite them some elementary bast fibres. 

 These quickly increase and become thickened, while around and 

 between them the thin-walled tissue equally enlarges by active cell- 

 division. When still more advanced, we see a bundle of lignified 

 spiral vascular cells of 3-7 elements, which appear to be pushed 

 somewhat toward the margin of the vascular bundle ; while oppo- 

 site to these, near the margin, is a bundle of bast fibres, the ele- 



JouRN. Q. M. C, No. 27. t . 



