84 MONOCOTYLEDONOUS STEM 



be distinguished. The exceptionally large vessels are a marked 

 feature of many other climbers besides the Vegetable Marrow. 



Stems of Monocotyledons (of which the Maize furnishes a 

 typical instance) usually exhibit a large number of bundles which 

 frequently appear scattered throughout the whole of the ground 

 tissue (Fig. 39), so that a definite cortex and pith cannot be 

 distinguished, i.e. there is often no sharply circumscribed stele 

 as in the ordinary Dicotyledon. The bundles, though differing 

 in certain details, show the same general structure, being col- 



FIG. 39. Photomicrograph of part of a transverse section of the stem of 

 the Maize (Zea mais). p., phloem ; p.x., protoxylem. 



lateral with xylem and phloem on the same radius and with the 

 protoxylems (p.x.) directed inwards. 



The phloem (Fig. 39, p.), which often has similar shining cell- 

 walls to that of the Sunflower, is an oval or rounded group of 

 tissue composed of sieve-tubes (Fig. 40, A, S.t.) and small com- 

 panion cells (C. ) . The latter are more rounded than is usual in 

 Dicotyledons, and this, combined with the absence of paren- 

 chyma, 1 leads to the remarkably uniform appearance of the 

 phloem. There is no cambium between the xylem and the 

 phloem, an important respect in which the Monocotyledonous 

 bundle usually differs from that of the Dicotyledon. The shape 

 1 Phloem-parenchyma is present in some monocotyledons. 



