554 Sinnott and Bailey. — The Origin and 
medullary rays, but to the transformation into parenchyma of the entire 
segments of xylem which are directly opposite the outgoing leaf-traces. 
Evidence in support of this view has been obtained principally from 
a study of certain woody and herbaceous members of the Rosaceae. 
Some typically woody forms in this family show signs of 5 compounding 5 , 
or the aggregation and increase in size of medullary rays, in the region 
just below the departure of the leaf-trace. Good examples of inter- 
mediate conditions between such a woody stem and the herbaceous type 
are not cited from other aerial stems, but almost entirely from the 
prostrate or subterranean rhizomes of several genera. In the case of the 
procumbent stem of Potentilla palustris , for example, there is ( 5 , p. 217) 
‘ in the segment of the xylem directly below the exit of each leaf-trace 
a decided lack of vessels and an agglomeration of rays 5 — the first step in 
the transformation of woody tissue into parenchyma. In the root-stock 
of other forms * a segment of the central cylinder is set off for some dis- 
tance below the passing out of a leaf-trace by the formation of large rays 
resulting from xylem parenchymatization. A progressive change of the 
xylem of this segment into parenchyma occurs upwardly towards the 
point of exit of the trace ; more and more of the secondary xylem 
being transformed, until only the primary tissue is left. This now sub- 
tends pure parenchyma, through which it passes out as the leaf-trace. . . . 
All that is necessary to produce a typical herbaceous stem from this 
structure is to reduce the size of the normally lignified xylem segments.’ 
And again, ‘ whole segments of the central cylinder in relation to the 
leaf-trace become transformed entirely into parenchyma with the excep- 
tion of the primary wood (which is the trace in the stem). Thus there is 
obtained a stem with alternating segments of typical xylem and paren- 
chyma, the latter with tiny groups of typical xylem elements on their 
centrad sides, or, in other terms, alternating large and very small bundles, 
the latter being leaf-traces.’ Attention is also frequently called to 
another feature in the development of the herbaceous type, the progres- 
sive ‘ localization of the primary wood or the breaking up into fewer 
and fewer separate strands of what was originally a continuous ring 
of primary elements. It is noted that when the xylem ring is very thin, 
the mere ‘setting off 5 of the leaf-traces, by conversion of segments 
on either side into parenchyma, is often sufficient to produce an herbaceous 
structure. 
It is evident that this hypothesis regards as the most important 
factor in the evolution of the herbaceous type of stem the so-called 
process of ‘ compounding 5 in connexion with the leaf-trace, by which 
the segments of normal secondary xylem opposite the departing traces 
are gradually converted into parenchyma, thus producing a stem with 
a few well-separated vascular bundles. The important steps in the 
