DIV. I 



MORPHOLOGY 



leaf is departed from, less and less deeply. In the longitudinal section of a stem 

 in Fig. 116 only the median bundle for each leaf (A, B, 0) and one lateral bundle 

 (a, b, c) are represented. In their further downward course the bundles gradually 

 approach the periphery of the stem, where they fuse with others. The number of 

 internodes whicli each bundle traverses varies, being greatest for the median 

 bundle. 



Structure of the Vascular Bundles ( 59 ). The bundles in the 

 stem are strands of tissue of circular or elliptical outline in cross- 

 section and always consist of xylem and phloem, i.e. are complete 

 bundles (cf. p. 67). The sieve-tubes are the most important com- 

 ponent of the phloem portion 

 and the water -conducting 

 vessels of the xylem portion 

 of the bundle. The bundles 

 are variously constructed in 

 different cormophytes, all 

 the types being represented xp - 

 in the stem (radial, concen- 

 tric, and collateral bundles). 

 These types are distin- 

 guished from one another 

 by the arrangement of the 

 strands of xylem and 

 phloem. 



In RADIAL vascular 

 bundles (Fig. 117; cf . also 

 Figs. 161, 163) there are 

 a number of strands of xylem 



, . , * FIG. 117. Radial vascular bundle from the stem of 



and phloem Which, as seen Lycopodium Hippuris. p.Pbloem; pp, primary phloem; 

 in a crOSS-Section of the *, xylem ; xp, protoxylem. (x 30.) 



circular bundle, stand side 



by side, alternating with one another. Seen from the side the vascular 

 strands run parallel to one another and to the longitudinal axis of the 

 part of the plant. The strands of xylem may meet in the centre of 

 the bundle and so constitute a star-shaped mass as seen in transverse 

 section. The ends of the rays are made up of the narrowest tracheides 

 (protoxylem), while the vessels towards the centre are always wider 

 (Fig. 117). The strands of phloem are situated in the depressions 

 between the rays, the narrow protophloem elements being at the 

 periphery. Radial bundles, though characteristic of roots, occur 

 relatively seldom in stems and are always solitary, as for example in 

 the stems of Lycopodium. 



In CONCENTRIC bundles a central strand of xylem or phloem is 

 surrounded on all sides by a cylinder of phloem or xylem. The 

 bundle may be distinguished as concentric with internal xylem when 

 the xylem is centrally placed, and as concentric with outer xylem 



