86 VARIATIONS IN STEM-STRUCTURE 



of the xylem as a whole is often roughly that of a Y, the stem 

 of which is occupied by a radial row of protoxylern -vessels (Fig. 

 40, A, Px.), whilst the arms are formed by a pair of large 

 metaxylem -vessels (Mx.). The phloem tends to be sunk between 

 the latter (cf. Fig. 40, A), but the degree of sinking varies con- 

 siderably in different stems, and in extreme cases the phloem 

 may even be completely surrounded by the xylem (as in the 

 Sweet Flag, Acorus calamus, Fig. 40, B). As usual, a certain 

 amount of wood-parenchyma occurs between the vessels. In 

 the stems of the Maize and many other Monocotyledons which 

 exhibit rapid elongation, the protoxylern- vessels undergo early 

 rupture and give rise to an irregular cavity (cf. p. 36) in which 

 remnants of the spiral thickenings are recognisable (Fig. 40, A, 

 P.O.). The bundles are commonly enveloped in a sheath of 

 small, thick-walled cells (often fibres, e.g. in the Maize, Fig. 40, A), 

 a feature also encountered in some Dicotyledons (e.g. Buttercup). 



The typical Monocotyledonous stem thus differs from that 

 characteristic of Dicotyledons in the large number, scattered 

 arrangement, and general form of the bundles, in the usual ab- 

 sence of a cambium, and in the detailed structure of the phloem. 

 Certain Dicotyledons (e.g. Buttercup and its allies), however, 

 show resemblance to Monocotyledons in the shape of the bundle, 

 in the absence of phloem -parenchyma, and the very feeble de- 

 velopment of the cambium. Moreover, the stem of the Dicoty- 

 ledon may exceptionally possess a relatively large number 

 of scattered bundles (e.g. Meadow Rue, Thalictrum), whilst that 

 of Monocotyledons may in its turn exhibit a ring-like arrange- 

 ment of the bundles (e.g. Black Bryony, Tamus, Fig. 41) and a 

 feeble development of cambium (cf. p. 117). 



In certain Dicotyledons which possess a normal ring of 

 vascular bundles additional ones occur in the pith (e.g. 

 Spinach, Water Dropwort, Oenanthe crocata) or cortex (e.g. Box, 

 Buxus sempervirens) . Such medullary and cortical bundles are 

 probably in most cases merely strands entering from the leaves 

 which have not yet taken their normal place in the vascular 

 cylinder (cf. p. 114). These bundles either show the normal 

 orientation (i.e. with the xylem towards the inside), or 

 they exhibit the reverse arrangement ; in many plants, moreover, 

 they consist of phloem only. 



