LINUM USITATISSIMUM 395 



adjacent bundles are separated from one another by medullary rays 

 one to several cells in width. As the stem matures, the activity 

 of the fascicular cambium, together with the initiation of an inter- 

 fascicular cambium, results in the formation of a continuous 

 cylinder of secondary xylem. In addition to this, there is a thick- 

 ening of the walls of the ray parenchyma, as well as of the medul- 

 lary parenchyma surrounding the primary xylem points, so that 

 connective tissue also contributes to the continuity of the mechani- 

 cal elements of the stele. The remaining cells of the medulla, aside 

 from the centrally located ones which disintegrate, are large and 

 thin-walled. 



The number of primary xylem elements in a single bundle is not 

 large, the protoxylem consisting of a few annular, annular-spiral, 

 and spiral types. As the stem matures, the rings in the annular 

 elements are usually distorted or partially obliterated, and the 

 spirals in the other types become much stretched as the inter- 

 nodes elongate. The metaxylem elements are scalariform and 

 reticulate, while the secondary xylem vessels are pitted with 

 vessel segments that commonly have oblique end walls. The 

 cells of the xylem parenchyma are much elongated axially and 

 have transverse end walls. The phloem consists of slender 

 sieve tubes, companion cells, parenchyma; and occasionally 

 phloem fibers, but the latter have no commercial importance. 

 (Fig. loi.) 



The Ontogeny of the Fiber — Initial Stages. — The fibers are 

 differentiated in the pericyclic region, and the initial stages in their 

 ontogeny take place just below the meristematic growing point of 

 the stem axis. In longisection, the pericyclic cells can be dis- 

 tinguished from the endodermal cells which lie immediately outside 

 them, since the former do not divide transversely as frequently as 

 the latter, the pericyclic cells compensating for the increase in the 

 length of the axis by cellular elongation rather than by division. 

 Early in ontogeny, however, the initial pericyclic cells do divide 

 periclinally several times so that this zone becomes a multilayered 

 region of large, thin-walled, elongated cells. 



Anderson Ql) has pointed out that the 



"enlargement phase of fiber development involves two types of increase: 

 an early, rapid, and extensive increase in diameter and length; and a 

 slow but perceptible increase in diameter, limited to certain cells, 

 that continues during the life of the plant." 



