§96 



A TEXTBOOK OF THEORETICAL BOTANY 



Comparative studies give good grounds for the opinion that the primitive 

 leaf trace system in Dicotyledons is one with three trace bundles leaving 

 three separate gaps in the vascular ring of the stem, the so-called trilacunar 

 type. Reduction to one is due in some cases to abortion of the lateral traces, 

 in others to fusion into one, which, however, may be three-lobed at the base. 

 The many-bundled type of trace is also derivative from the three-trace type, 

 which is frequently found in the seedlings and youngest leaves of plants with 

 complex traces. Even in Monocotyledons the three-trace type is found in 

 many small and slender species and in the young leaves of others, and it 

 appears to be primitive in this group also. 



In Monocotyledons the leaf base is broad and surrounds the stem, even 

 in the embryonic state. It may contain thirty to forty trace bundles, which 



CURRENT LEAVES 



LEAF TRACES 



LEAF TRACES 



SCALE TRACES 



YOUNGEST ROOT 



OLDER ROOT 



Fig. 877. — Convallaria majaJis. Diagram illustrating the course of vascular bundles 



in the rhizome. {After Euker.) 



enter the stem simultaneously all round the periphery. Each leaf primordium 

 is quickly pushed outwards away from the apex by the new encircling 

 primordia which follow it, with the consequence that its traces become acutely 

 flexed at the node, bending outwards from a central position into the leaf 

 base (Fig. 877). In Monocotyledons with extended internodes the bundles 

 follow a course downwards which is similar to that in Dicotyledons, tending 

 gradually outwards towards the point of anastomosis with older bundles, 

 which appear to be usually those of the next leaf vertically below the leaf 

 considered, i.e., on the same orthostichy (see Chapter XXII). At the nodes 

 the number of anastomosing bundles is so great that it creates a nodal plexus 

 of intermingled bundles which may extend right across the stem, and is 

 increased by the insertion of numerous traces from the axillary buds. 



It must be admitted that the course of the bundles in even a simple 

 Monocotyledon is not yet known with sufficient accuracy to warrant positive 

 statements, while in those species with contracted stems the complexity 

 and irregularity defies analysis. The whole question of vascular develop- 





