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Herbaceous Type in the Angiosperms. 
obsolete only in small bundles. The reduction in size of the herbaceous 
stem, together with its slight development of xylem and lack of necessity 
for storage parenchyma, removes the demand for cambial activity. The 
cambium then disappears, first, naturally, in those regions where it gives 
rise to tissue no longer typically fibro-vascular. Its complete disappearance 
finally occurs when the bundles themselves have become much reduced. 
Summary. 
The prevailing view concerning the origin of the solid woody cylinder 
of the Angiosperms is, that its formation results from the fusion of a group 
of originally separate bundles ; that this is accomplished by the extension 
of the cambium arising within those bundles across the interfascicular tissue, 
thus completing a cambium ring which subsequently develops the continuous 
cylinder. Instead of this, however, a reverse process seems to have occurred. 
A primitively solid cylinder has been reduced and dissected to form the 
type characteristic of dicotyledonous herbs — a ring of small, separate 
bundles. In favour of this view there is much evidence, both direct and in- 
direct. The anatomical structure of fossil forms and parallelism in develop- 
ment in cryptogamic groups still living points in this direction. Direct 
development proof is supplied by some of the more herbaceous members 
of the Rosaceae, particularly the herbaceous perennials. 
Further, there has apparently been a complete misunderstanding of 
the structure of some of the plants chosen to illustrate in textbooks the 
evolutionary development of the stem, as, for example, the case of Quercus . 
Diagrams of other plants of somewhat similar structure do not always 
indicate the particular plant they represent, and err perhaps too much 
on the diagrammatic side. Two genera very commonly illustrated in this 
connexion, Aristolochia and Clematis , seem at first glance to offer evidence 
in support of the older theory. But consideration of them as members of 
a class that is presumably highly developed, the vines, comparison with 
each other and with other chosen illustrative plants, together with a careful 
developmental and anatomical study, shows that whatever light is to be 
obtained from them in these matters aids in the formation of the opinion 
that the fascicular type of central cylinder has been derived from the 
unbroken woody cylinder of the lower Dicotyledons. 
Herbaceous plants, especially annuals, are clearly very efficient forms. 
They have probably arisen as an adaptation to modern conditions, and their 
adaptability, together with rapid and greatly increased reproduction, has 
much facilitated their evolutionary progress. 
The impetus for so great a change seems, in large part at least, to have 
been given by the leaf-trace. This has been effected by the transformation 
of small or large masses of secondary xylem into storage parenchyma in 
those segments of the woody cylinder directly related to the leaf-trace. 
