250 



PTERIDOPHYTA. 



[CH. 



succeeded internally by the general ground- tissue of the stem. 

 The xylem parenchyma next a carinal canal is succeeded 

 externally by phloem tissue, consisting of short protoplasmic 

 cells and longer elements, without nuclei and poor in contents ; 

 the latter may be regarded as sieve-tubes. On either side of 

 the phloem, the xylem occurs in two separate bands or groups 

 of annular and reticulately thickened tracheids. In some species, 

 e.g. Equisetum xylochaetum Metten.^ and E. gigmiteum^ L. 

 a native of South America, the xylem has the form of two 

 bands composed of fairly numerous tracheids, but in most 

 species three xylem tracheids occur in small groups, as shown 

 in the figure of E. maximum (fig. 52, D). In the shape of the 

 vascular bundle, and in the formation of the carinal canal, there 

 is a distinct resemblance between the vascular bundles of 

 Equisetum and those of a monocotyledonous stem. These 

 collateral stem-bundles of xylem and phloem traverse each 

 internode as distinct strands, and at the nodes each strand forks 

 into two branches (fig. 54, A), which anastomose with the 

 alternating bundles passing into the stem from the leaf-sheath. 



Ik e 



k>j^<7Wv^ 



A B 



Fig. 54. A. Plan of the vascular bundles in the stem of an Equisetum; 

 b, branches passing out to buds (after Strasburger) ; I, vascular strands 

 passing to the leaf-segments. B. Longitudinal section through a node of 

 E. arvense L. (after Duval- Jouve; x 20). Explanation in the text. 



Milde (67) PI. xix. fig. 8. 



ihid. PI. XXXI. fig. 3. 



