Dispersal of Herbaceous A ngio sperms. 597 
often been supplemenated by widening of the medullary rays and, more 
rarely, by an increase in the development of secondary parenchymatous 
tissue between the bundles. In the case of many families, however, the 
herbaceous members have a perfectly continuous ring of vascular tissue. 
The herbaceous stem in its general character is very much like the first 
annual ring of a related woody plant. 
6. Evidence from phylogeny shows that the more primitive groups of 
Angiosperms, and the lower seed-plants from which they have presumably 
been derived, are composed overwhelmingly of woody plants. In those 
particular orders or families, also, which comprise both herbs and woody 
plants, and in which it is possible to determine with reasonable certainty 
the relative antiquity of the various members, the primitive types are found 
to be much more woody than the recent ones. In more than half of the 
families of Dicotyledons, there are no herbaceous species, and the few 
families which are entirely herbaceous are almost all insectivorous plants, 
water plants, parasites, or monotypic families, and hence can lay no claim 
to great antiquity. Woody plants are much more abundant among the 
Archichlamydeae than among the Metachlamydeae. 
7. Since the Monocotyledons probably arose in very ancient times 
from the primitive Dicotyledons as an aquatic herbaceous adaptation, 
woody forms in this group are to be considered as recent rather than 
as primitive. 
8. Evidence from phytogeography also supports the contention that 
the most ancient Angiosperms were woody. There is a great preponderance 
of herbs in temperate regions and of woody plants in the tropics. The 
latter climate probably approaches more nearly to that under which 
Angiosperms first appeared. 
9. Herbs have a short life-cycle, and are therefore able to survive 
periods of cold underground or in the form of seeds. Their great develop- 
ment in temperate regions has probably been in response to the progressive 
refrigeration of the climate during the course of the Tertiary. 
10. A considerable number of plants now occur only in eastern Asia 
and eastern North America. They form a flora which probably approaches 
that of the north temperate zone in preglacial time. It is composed of 
about an equal number of herbs and of woody plants. 
11. The advent of the glacial period resulted in the extermination 
of a large part of the vegetation of the north temperate zone, but this 
extermination was proportionately much greater among woody plants than 
among herbs. The present flora of Europe contains a decidedly smaller 
element of woody plants than does that of corresponding temperate North 
America because of the inability of the plants in the former region to 
migrate southward on the approach of the glaciers. 
12. The present flora of Northern Europe has apparently received but 
S s 2 
