THE HIGHER CRYPTOGAMIA. 215 



very woody, like bast-cells. Their very thick walls, 

 which are pierced by pits or canals, assume a brown 

 colour throughout. Thin sections of them are of a 

 beautiful golden yellow ; when seen in a mass they 

 are almost black. The axile region of the stem thus 

 appears, even to the naked eye, to be distinctly separated 

 from the bark by a thick hard sheath of vascular bundles, 

 which has a fissure-like longitudinal opening only on each 

 of the two sides parallel to the outer longitudinal bands of 

 the stem (PI. XXX, fig. 3). One of these fissures is 

 often closed by an amalgamation on one side of the two 

 halves of the sheath of vascular bundles. The upper half 

 of the sheath is tolerably flat ; the lower one has the form 

 of a furrow. During the transformation of the parenchy- 

 matal cells of the end of the stem into bast-cells, air- 

 bubbles are formed (PL XXX, fig. 12), between the walls of 

 the latter, in the interior of small irregularly- defined inter- 

 cellular cavities. These air-bubbles disappear when the 

 thickening of the walls commences. 



The outermost cellular layers of the bark also assume a 

 deep brown colour, but without becoming prosenchy- 

 matous, and without any material thickening of their walls. 

 Those portions only of the tissue which pass towards the 

 lateral longitudinal ridges do not assume this brown colour, 

 which extends to the depth of one-eighth of a line into the 

 cortical tissue. The portions just mentioned, like the 

 parenchyma of the interior of the stem, remain of a dazzling 

 white : they contain starch, and their intercellular cavities 

 are filled with air. Here and there in this tissue, and 

 sometimes also in the brown-coloured outer cortical layer 

 spindle-shaped groups of combined cells become trans- 

 formed into thick-walled bast-cells, similar in all respects to 

 those of the sheath of vascular bundles.* 



As the vascular bundles in the growing stem become 

 more complicated, so also do those in the stipes of the 

 fronds. As in the first, so also in the other fronds of the 

 young plant up to the twelfth, the vascular bundles unite to- 



* Molil objects to these cells being called bast-cells (' Vermischte Schriften,' 

 p. 136), but in their form and mode of development they agree exactly with the 

 bast-cells of phfeuogams. 



