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A TEXTBOOK OF THEORETICAL BOTANY 



The closed bundle which is characteristic of the aerial stems in Mono- 

 cotyledons is small and has relatively few elements. It is sometimes described 

 as the Y-type from the characteristic grouping of the xylem elements. The 

 protoxylem, which is often crushed and obsolete at maturity, occupies the 

 tail of the Y, and the arms are formed by, or end in, two very large vessels, 

 between which lies a group of small lignified tracheids. Above this is the 

 compact phloem, consisting of regularly arranged sieve tubes and companion 

 cells with a few parenchyma cells at the sides. Another monocotyledonous 

 type has a still simpler xylem, formed of one or two narrow protoxylem 



Xylem 



Phloem 



Fig. 875. — Convallaria majalis. Transverse section of a 

 single centric vascular bundle from the rhizome show- 

 ing phloem surrounded by a ring of xylem. 



elements and one extremely large vessel in the centre of the bundle. This 

 type of bundle is formed mostly in monaxial Monocotyledons with very 

 large, rapidly growing leaves, e.g., Banana, and the extra large vessels are 

 probably correlated with the need for quick development of the water- 

 conducting capacity of the stem. The concentric type of bundle is considered 

 to be the most advanced type in the Monocotyledons and has probably been 

 derived from one of the collateral types. 



As is well known, the arrangement of the vascular bundles in Dicotyledons 

 is typically in a single, wide ring, while in Monocotyledons they are typically 

 dispersed, seemingly at random, across the transverse section of the stem. 

 In the Dicotyledon the orientation of the vascular tissues is constantly with 

 the protoxylem innermost (the endarch position), but in Monocotyledons, 

 although the general tendency is the same, there is less uniformity, especially 

 in the bundles near the centre. 



The parenchymatous tissue enclosed by the ring of bundles in Dicotyledons 



f 



