R. BRAITHWAIPE ON THE HISTOLOGY OF PLANTS. 297 



bast part of the stem, in groups separated from each other by the 

 other elements ; in Finns they are wanting, or scattered in the 

 other elements. 



Bast Parenchyma. — The parenchyma cells in Cycadece alternate 

 with the bast vessels without any regular order in the groups, 

 bounded internally and externally by bast fibres and laterally by 

 pith rays. In the Vascular bundles of Conifers they are more 

 regular, so that in Abietinece and Araiicarice, a certain serial 

 arrangement in a tangential direction is evident, while in Cvpres- 

 sinece and Taxinea?, they stand between the rows of bast vessels. 

 In all the contents are starch, and in Cycas, and more rarely in 

 Pinus, crystals are also present. 



Bast vessels. — The tube cells in Cycads are placed irregularly 

 among the parenchym cells, but in Abietinece they are in radial 

 two or more celled rows, interrupted by a row of parenchym cells ; 

 in Cupressinece and Taxinece they stand between the bast fibres and 

 bast parenchyma, the latter forming a central row. 



The Inteimediate Tissue. — This also agrees with that of other 

 trees, and in CycadecB the primary rays as well as the bundle rays 

 are only faintly thickened, and not lignified, while in Conifers the 

 thickening is moderate, and lignification takes place in the whole 

 cellulose case, except in the membrane closing the pore canals. 

 The medullo-cortical rays of Cycads consist of several rows, and 

 become wedge-shaped toward the outer part of the wood, to widen 

 again in the bark ; Conifers have only one or at most two- rowed 

 rays, and possess in the wood and bast parts an ecjual number of 

 cell rows. The bundle rays both of Cycads and Conifers are only 

 small and of single rows, though here and there in the Larch, 

 Pinus Canariensis, &c., rays of two or three cell rows occur. 



Air, or Peculiar Sap-canals in the Vascular Bundles. 



In the tissue of the Vascular bundles of Vascular Cryptogams, 

 Mono and Dicotyledons, we find in certain genera peculiar re- 

 ceptacles like those occurring in pith and bark, which carry air 

 or excreted matters, and known as air passages or sap recep- 

 tacles. We find air passages only in the Vascular bundles of 

 EquisetacecB, and certain Monocotyledons, and in the former they 

 originate in the stem by resorption of a string of vessels and 

 parenchyma, in the root partly by resorption of a large central 

 spiral vessel ; in the latter they owe their origin to a similar 



