28 . BOTANICAL GAZETTE (raed ane 
reduction in or complete loss of secondary growth is physiologically 
advantageous to the plant. 
An incidental and probably less essential modification in extreme 
herbs is the concurrence of medullary and amphivasal strands. 
These are apparently the result of the entry of very numerous 
bundles at the nodes, a feature of many advanced and angio- 
spermous herbs. The resulting difficulties of accommodation 
are most readily overcome by the scattering and fusion of the 
crowded strands. In lower types the amphivasal and medullary 
bundles are the consequence of the fusion of facing cauline bundles 
of axis and lateral branch. In the highest herbs the foliar traces 
are concerned in the formation of medullary strands and in the 
scattered distribution of bundles throughout the transverse sections 
of the stem. 
Summary 3 
1. Herbaceous Dicotyledons have developed from arboreal 
dicotyledonous types by the formation of Storage rays about the 
leaf traces. 
2. In the more primitive herbs the foliar rays are shallow 
longitudinally, but of considerable radial depth. 
3. In higher herbaceous Dicotyledons the foliar rays lose in 
radial dimensions as a result of the thinning of the woody cylinder, 
but this loss is largely compensated for by their increasing vertical 
extension, which often carries them through several internodes. 
4. In the higher herbaceous Dicotyledons the foliar traces tend 
to multiply in number with the increased efficiency of the leaf. 
5. Another important development in advanced dicotyledonous 
herbs is the progressive disappearance of cambial activity in the 
foliar trace, which often, in spite of this, is larger in size than the 
bundles of the stem. 
6. The increase in number and importance of the foliar traces, 
as well as the greater relative importance of secondary axes in 
high herbs, leads to the crowding of strands at the node, which in 
turn results on the one hand in scattered distribution of the bundles 
in the stem, and on the other to formation of amphivasal strands. 
. The disappearance of secondary growth in foliar traces of 
advanced dicotyledonous herbs appears to be explainable on the 
grounds of physiological advantage. 
