442 Worsdell. — Origin and Meaning of Medullary 
to the sclerotic tissue covering the xylem of the ring-bundles. This is the 
only case in which medullary strands have been observed in this tribe. 
S. clivorum, Maxim. 
Leaf. 
The petiole contains a large number of variously orientated and sized 
bundles scattered indiscriminately throughout the tissue (Fig. 18). The 
petiole of S. Petasites does not show this scattered disposition, but a more 
reduced type of structure which has evidently been derived therefrom, and 
is much more primitive than that of the stem. 
Fig. 17. Gundelia Tournefortii Segment Fig. 18. Senecio clivorum. Petiole, 
of cylinder of stem, showing scattered medul- showing extreme scattered disposition of the 
lary phloem-strands (?nps). x 4. bundles, x 5. 
Petasites officinalis, Moench. 
Peduncle {Male Plant). 
The scattered disposition of the bundles composing the main vascular 
system is very pronounced, especially in the upper part of the organ 
(Fig. 19). The presence of small, rudimentary, and abnormally orientated 
strands suggests the derivation from an even more pronouncedly ‘ Mono- 
cotyledonous ’ type of structure. 
Petiole. 
This organ exhibits one of the best examples known of the primitive 
scattered distribution of the bundles, this feature being directly connected 
with the large size of the leaf, adequate space being present for the preserva- 
tion of the original structure. There is a complete vascular ring with great 
numbers of bundles of varying size and orientation, some of which are rudi- 
mentary, distributed throughout the ground-tissue (Fig. 20). In the basal 
region, where the central lacuna has greatly dwindled in area, further strands 
of rudimentary structure appear nearer the centre. 
