178 XIII. THE STEMS OF PTERIDOPHYTES. 



parallel to one another. On the one side they are somewhat 

 concave, on the other proportionately convex ; and we can 

 determine, if we take note of the natural position of the creeping 

 stem upon the earth, that the bands appear parallel to the 

 surface of the earth, and with the concave side always turned 

 upwards. The small steles of the leaves, after they have entered 

 into the central cylinder, join on to the edges of a xylem band, 

 just as in the ferns. The xylem bands not infrequently anasto- 

 mose, an example of which can be seen in the lower bands of the 

 sketch (Fig. 70). In the erect stems of Lycopodium Selago the 

 whole of the xylem bands are, in fact, combined into the form of 

 a star. The xylem bands are surrounded by a single layer of 

 thin -walled, narrow-cavitied cells, which, as in the ferns, we 

 can designate wood-parenchyma cells. Midway between the 

 bafids formed by the wood lie cells with white, strongly refractive 

 walls ; they have narrow cavities, but a median row is distin- 

 guished by somewhat broader cavities. These] form the bast ; 

 the larger elements in this latter are the sieve-tubes (v). In 

 specially favourable cases of safranin staining, the walls of the 

 sieve-tubes are rose-red, while the other elements of the bast 

 remain colourless. At the edges of these bands of sieve-tubes 

 the protophloem elements are distinguished by the narrowness 

 of their cavities. At the inner limits of the cortex the central 

 cylinder (gamostele) can be easily broken away in section-cutting. 



Longitudinal sections show us : most externally, the epidermis ; 

 then, the broad cortical cells running obliquely towards it ; 

 further, the sclerenchyma fibres of the outer cortical sheath ; 

 after this the inner cortical sheath of elongated parenchyma ; 

 the innermost cortical sheath with white, thicker walls, and end 

 walls situated obliquely ; the scalariform trachei'des, and the 

 narrow, in part very greatly stretched, annular and spiral ele- 

 ments of the protoxylem ; finally, also, the elements of the bast. 

 These last consist of very long cells, the end walls of which are 

 more or less oblique. With the aid of aniline blue it is possible, 

 though very difficult, to recognise the comparatively small, 

 oblique sieve-plates. Only the broadest elements in the bast 

 are sieve-tubes, the much more numerous narrow elements, 

 filled with refractive granular contents, form the protein-con- 

 taining bast-parenchyma. 



Stem of Equisetum. There are, however, also, Pteridophytes 

 whose stem structure shows collaterally-constructed vascular 



