498 



MOSSES AND FERNS 



CHAP. 



conspicuous as the section is examined lower down. The 

 formation of this lacuna is similar to that in the capsule of the 

 Bryineae, and, as there, the central mass of tissue is connected 

 by rows of cells with the outer tissue. These rows of cells 

 (trabeculae) are at first composed of but a single cell, but later 

 by tangential walls become slender filaments by which the 

 vascular cylinders are suspended in the large lacuna which 



occupies the centre of 

 the stem (Fig. 262, t). 

 According to Stras- 

 burger -^ both the trabe- 

 ^ "' culae, which are usually 



regarded as endodermal, 

 and the pericycle, are of 

 cortical origin. 



The fully -developed 

 bundle in S. Kraussiana 

 (Fig. 262, B) shows a 

 pericycle composed of a 

 single layer of rather 

 large cells, within which 

 lies the phloem, which 

 completely surrounds the 

 xylem, as in the Ferns. 

 The sieve -tubes in this 

 species form a single 

 circle just inside the peri- 

 cycle, but according to 

 Gibson " are absent op- 

 posite the protoxylem. 



Fig. 262. — Cross-section of a fully-developed stem oi S. 



Kraussiana, showing the two vascular bundles sus- rle StatCS tnat tnCre IS 



pended in the large central lacuna by means of the Kiif p ^incrlp DTOim Ctf 

 trabeculae (J), X 75 ; B, a single vascular bundle, & Jr 



X450 ; X, X, scalariform tracheids ; j, s, sieve-tubes. prOtOXylcm clcmentS 



here, but my own ob- 

 servations lead me to think that there are two, as Russow 

 affirms is the case. The origin of the protoxylem was 

 not traced, but the appearance of the mature bundle in the 

 specimens examined (Fig. 263, B) points to this conclusion. 

 The protoxylem is made up of small spiral and annular 

 tracheids, the metaxylem (secondary wood) of larger scalari- 

 •^ Strasburger (7), p. 457. - Gibson (2), p. 176. 



