THE ANGIOSPERMAE : STEMS 



889 



is the medulla or pith, but in monocotyledonous stems there is no corre- 

 sponding region, and the parenchyma among the bundles is generally known 

 as the ground tissue or conjunctive tissue. 



Departures from these typical arrangements will be dealt with later. We 

 must now consider the bundle systems in three dimensions in order to get 

 some idea of vascular architecture. 



In Dicotyledons the leaves have usually a narrow base of insertion on the 

 stem. The number of leaf traces is therefore nearly always small and may 

 be only one. These traces differentiate downwards 

 from the leaf base and intercalate themselves be- 

 tween the traces of lower leaves (Fig. 876). In this 

 way the trace of a young leaf may reach downwards 

 through several internodes. Eventually, however, at 

 a node, it either becomes united laterally to the 

 adjacent bundle in the ring, or it forks and unites 

 to the bundles on each side of it, producing " syn- 

 thetic " bundles. The distance it reaches down- 

 wards before this happens depends on the leaf 

 arrangement on the stem, since it appears that the 

 traces to which a given leaf unites itself are 

 those of the leaf vertically below it in the 

 leaf spiral. At every node some anastomosis of 

 traces takes place, so that there is at these 

 levels a considerable amount of vascular linkage, 

 while in the internodes the bundles remain 

 separate. 



The lateral traces of the leaf, where such exist, 

 are close to the median trace at the leaf base, but 

 as they go downwards they are separated by the 

 intercalation of traces from higher leaves, until they 

 may lie far apart. Where there are several traces to 

 each leaf they will thus come to be spaced out 

 where they enter the ring, and in types with many 

 traces, such as Liriodendron, they may occupy points 

 all round the ring before they finally join the ring 

 bundles. The lateral traces join the ring at a higher level than the median 

 trace, which means, in terms of apical development, that they do not 

 differentiate until several plastochrons later than the median trace, which is 

 always the strongest and largest and goes farthest down the stem. While 

 the trace differentiates downwards, the period of growth available for 

 differentiation gets shorter in each successive internode and the trace will 

 therefore contain less and less primary xylem and more and more secondary 

 xylem, until it is finally merged in the general mass of secondary xylem. 

 The phloem generally accompanies the xylem, though peripheral portions 

 of phloem may branch off independently and join up with others far above 

 the level at which the trace as a whole finally joins a synthetic bundle. 



Fig. 876. — Vicia faba. 

 Portion of a stem in- 

 cluding a node cleared 

 in lactic acid to show 

 the course of vascu- 

 lar bundles. Trace 

 bundles pass out into 

 the petiole and axillary 

 shoots on the right. 



29 A 



