596 
Sinnolt and Bailey. — The Origin and 
become adapted to adverse conditions, are therefore the most hardy and 
aggressive types of vegetation and have consequently been able to invade 
successfully all regions of the globe. 
Summary. 
1. The problems under discussion in the present paper are the relative 
antiquity of herbs and woody plants ; and the manner in which the more 
recent of these two types has been developed and become dispersed. 
2. Palaeobotanical evidence shows that the ancient representatives 
of several of the lower orders of vascular plants were woody, although their 
living members are herbaceous. Almost all the earliest Angiosperm remains 
are also of woody plants. The geological evidence in this case, however, 
although seeming to indicate that the primitive members of the phylum 
were woody, cannot be regarded as proving the point conclusively, for the 
leaves of trees and shrubs are probably more likely to escape destruction 
and to be preserved as fossils than are the more tender ones of most 
herbaceous plants. 
3. Evidence from anatomy indicates that in all groups of vascular 
plants which possess secondary growth the cambium appeared originally as 
a uniform and continuous layer ; and that its distinction into a ‘ fascicular 
portion, producing wood, and an ‘interfascicular ’ portion, producing paren- 
chyma, has been of comparatively recent occurrence. The continuous 
woody ring which characterizes the stems of all trees and shrubs is there- 
fore to be regarded as more primitive than the many-bundled herbaceous 
type. The structure of woody stems also displays primitive histological 
characters often absent in those of herbs. 
4. Jeffrey and his school have maintained that the herbaceous stem 
was derived from the woody type by a conversion into parenchyma of 
whole segments of the central cylinder directly opposite the bundles which 
were to depart as leaf-traces, and that these segments constitute the inter- 
fascicular parenchyma between the bundles of the herbaceous stem. The 
writers dissent from this theory, both on the ground that evidence in its 
support is derived not from the aerial parts of the plant but almost entirely 
from prostrate stems or subterranean rhizomes, which are nearly always 
much modified in structure, owing to their function as food reservoirs ; and, 
more particularly, on the ground that it is not in harmony with a great body 
of anatomical facts. In practically all families of herbs, the interfascicular 
parenchyma is never subtended by a tiny leaf-trace bundle of protoxylem, 
but always abuts directly on the pith tissue between the strands of primary 
wood. 
5. The chief factor in the development of the herbaceous stem seems 
to have been a simple decrease in the activity of the cambium. This has 
