444 Worsdell. — Origin and Meaning of Medullary 
It is an interesting fact, by no means confined to this genus, that in 
the receptacle or axis of the capitulum the bundles of the ring become 
completely amphivasal like those of the pith in the peduncle and stem. 
This the writer regards as the retention of a primitive feature, there having 
been no call for its modification in this part of the plant-axis as there has 
been in the case of the vascular ring of peduncle and stem. It cannot be 
explained as a necessary structure for the supply of the florets, but has 
been retained through not being adversely adapted to this purpose. 
Leaf. 
In the petiole and midrib there is a simple, regular arc of alternately 
large and small (some very minute) bundles (Fig. 23). Apart from the 
great disparity in size of the bundles, which is a primitive character, the 
structure is an advanced (reduced) one as compared with that of the stem, 
the bundles of the arc being regularly alined and there being a complete 
absence of medullary strands. 
R. digitata, Mill. (. R . laciniata , L.). 
Stem and Peduncle . 
The vascular ring shows the same features as in the last species. The 
cortex is extremely narrow. Medullary bundles are completely absent, 
although the lacuna is not more extensive than it is in the last species 
(Fig. 21). 
Receptacle of Capitulum . 
The vascular ring consists of three series of bundles of which the 
structure is amphivasal. The innermost are the largest. There are no 
central medullary bundles. 
Leaf. 
The structure is the same as in the last species. 
In R. calif or nica, A. Gray, R. occidentalism Nutt., R. Newmani , Loud., 
R. columnar i$m Sims, R. peduncularis , Ton*, and Gray, and others, medullary 
bundles are completely absent, and the leaf has a simple arc of bundles. 
Echinacea purpurea, Moench. 
Stem. 
In the higher part of the stem there occur in the centre of the pith six 
or seven small bundles of amphivasal structure, consisting of soft bast 
entirely enclosed by xylem-fibres ; one or two small vessels or tracheides 
were noticed in one bundle ; one or two bundles are quite rudimentary, 
consisting of xylem-fibres only (Fig. 24). Owing to lack of material the 
