45 6 Worsdell. - — Origin and Meaning of Medullary 
stem and leaf are really one and indivisible, the former being built up of 
leaf-bases). The theory undoubtedly receives confirmation from a study of 
the vascular system in other Natural Orders. 
In Polygonaceae the genus Rumex contains some species which possess 
in the stem a medullary system of strands, always in a more or less imper- 
fect, rudimentary form ; while in the stem of other species it is completely, 
or almost completely, absent. But in the petiole of the leaf of all species, 
with the possible exception of the small, reduced R. Acetosella , medullary 
bundles occur, not in an imperfect, rudimentary, but in a very well- 
developed condition. This uniformity in the vascular structure of the 
leaf as compared with the variableness and inconstancy of that of the stem 
is strongly in favour of the view that the vascular structure of the leaf repre- 
sents, of the two systems, the more ancient type, that, viz., in which a well- 
pronounced, scattered system of bundles occurred in both stem and leaf. 
In Umbelliferae there are a large number of cases in which, where the 
stem exhibits no trace of medullary strands, they are well developed in the 
petiole. The markedly imperfect, rudimentary nature of the medullary 
system of the stem, in the majority of cases where it occurs, clearly shows 
this feature to be an ancestral, vestigial one, which, in many species, has 
been preserved in its pristine condition in the more conservative foliar 
organs. 
The adaptive theory, as set forth on an earlier page, cannot account 
for the very frequent presence of rudimentary, imperfect strands both in 
stem and petiole. If they represent a comparatively recent acquisition 
resulting from the congested type of inflorescence, then the very widely- 
diffused presence in Compositae of the scattered system (or medullary 
strands) in an imperfect condition of development can with difficulty be 
accounted for and is inconsistent with its having been recently acquired ; 
there should be, one would think, some very definite explanation thereof 
forthcoming. The writer, however, knows of no adequate explanation of 
the phenomenon from the point of view of the theory opposed to his own. 
On the writer’s view, there must be one single theory to cover all the 
facts of the case, which is essentially the same for all the Natural Orders 
concerned ; a single theory to account for the medullary system in both 
stem and leaf in whatever condition, perfect or imperfect, it may occur. 
The writer is acquainted with but one theory which will accomplish 
this, viz. that the Compositae, as also the great majority of Dicotyledonous 
Natural Orders, are derived from a c Monocotyledonous ’ or geophytic stock 
in which the grandifoliate character prevailed, the leaf dominating the stem, 
and the latter being primarily built up of leaf-bases. The two organs, stem 
and leaf, would be thus in reality one and indivisible, and their vascular 
structure identical, such as we see in the typical Monocotyledon of the 
present day. 
