Phloem in the Stems of Dicotyledons . II. 443 
It would be difficult to attribute an adaptive significance to the vascular 
structure of this organ ; it must, therefore, be aboriginal. 
Helianthoideae. 
Rudbeckia maxima, Nutt. 
Stem and Peduncle . 
There are two well-marked features of the vascular ring which strongly 
suggest its derivation from a primitive scattered system of bundles, viz. 
(i)'the extreme irregularity of alinement of its component bundles, which 
are at very variable radial distances from the centre of the pith, and (2) 
Fig. 19. Pctasites officinalis. Peduncle of male plant, showing scattered disposition 
of bundles, x 5. 
the great difference in the size of the bundles, some being very small and 
even rudimentary, and irregularly orientated. 
There are very well developed medullary bundles, of perfect amphi- 
vasal structure, forming a ring about half-way between the ring and the 
central lacuna. Nearly all are seen to be branching into two or three 
(Fig. 22). In the peduncle some of the medullary bundles occur nearer the 
ring, and some are extremely rudimentary. 
If traced downwards the medullary bundles are all seen to die out in 
situ in the region of the primary node, just above where root-structure 
begins. If traced upwards they, in some plants, disappear in the peduncle, 
either dying out in situ or passing into the ring. In other plants they persist 
into the receptacle of the capitulum, the same fate befalling them there . 1 
1 In the uppermost region of the peduncle, and also in the receptacle, the lacuna has disappeared 
and, as a consequence thereof, medullary bundles occur in the centre of the pith. 
