STRUCTURE OF COLLATERAL BUNDLES. 



329 



cells of the xylem. In the stem of Rhapis flabelliformis most bundles have a phloem 

 which is somewhat crescent-shaped in cross-section ; it is surrounded by a thick 

 sheath forming a bundle of fibres, which projects strongly towards the outside; round 

 the small xylem, consisting of a few vessels surrounded by thick-walled cells, the 

 sheath is feebly or not at all developed. In some of the inner bundles a ridge-like 

 projection about three layers thick passes from the sheath to the thick-walled cells of 

 the xylem through the middle of the phloem, severing the latter into two symmetrical 

 halves. Essentially the same phenomenon appears much more conspicuously in the 

 stem of Calamus. The xylem, as already stated at p. 323, always shows some narrow 

 spiral vessels, and outside these a very large pitted vessel ; around and between them 

 lie thick-walled elongated cells. In the smaller peripheral bundles the single phloem, 

 semicircular in cross-section, lies on the outside of the pitted vessel, and is surrounded 

 by a strong sheath of fibres, which is continued round the xylem. In most bundles 

 the phloem is divided by a broad continuation of the sheath, extending up to the 

 pitted vessel, into two portions lying right and left of the latter. Each of these 

 portions consists of some large sieve-tubes (comp. p. 176) standing in a row parallel 

 to the circumference of the vessel, together with the accompanying cambiform cells. 

 This row also may once more be interrupted by sclerenchyma, so that individual sieve- 

 tubes with their accompanying cells stand isolated in the sclerenchyma of the sheath \ 

 In the mature leaf of species of Pandanus, the phloem appears at the first glance 

 to be wholly absent even in thick bundles, and in its place a strong narrow strand of 

 sclerenchymatous fibres seems to border immediately on the xylem-. More accurate 

 investigation shows isolated sieve-tubes between the sclerenchymatous fibres. In 

 younger bundles they are easier to 

 find — perhaps also more numerous ; 

 it seems as if here, as in other bun- 

 dles, sieve-tubes present at an early 

 period were afterwards crushed by the 

 neighbouring cells and rendered un- 

 recognisable, but this has still to be 

 investigated. 



As the parts belonging to and bor- 

 dering on the vascular bundle must 

 in fact be considered in connection 

 with one another, it seems more ex- 

 pedient here, in conclusion, to collect 

 together some figures referring to 

 collateral bundles, explaining them 

 with reference to the paragraphs 

 above, than to insert them among 

 the preceding descriptions of the in- 

 dividual parts. 



Fig. 149. Cross-section through a 

 mature internode of Equisetum 



Fig. 149. 



* [Compare Kny, Verhandl. d. Bot. Ver. Prov. Brandenburg. Bd. XXIII, 18S1, pp. 94-109; 

 also his Bot. Wandtafeln, V.] 



2 Compare Meyen, /. r. (p. 323). — Van Tieghem, Ann. Sci. Nat. 5 ser. VI. p. 197. 



