550 Sinnott and Bailey. — The Origin and 
Isoetaceae, and Equisetaceae, are entirely herbaceous but vestiges of a 
cambium in Isoetes and Eqiasetum point to the previous activity of such 
a tissue. In these orders the herbaceous forms have, without much question, 
been derived from ancestral woody types. 
Among Angiosperms, palaeobotanical evidence seems at first sight to 
point certainly to the conclusion that the arborescent members of the 
phylum are more ancient than the herbaceous ones, for the overwhelming 
majority of fossil Angiosperms are related to forms which to-day are always 
trees and shrubs. The objection frequently raised to this conclusion, and 
with good reason, is that the absence of herbaceous leaf-impressions is 
not due to the absence of herbs in ancient times, but rather to the fact 
that leaves of such plants are generally much more delicate and less apt 
to be preserved in a fossil state than are the tougher ones of trees and 
shrubs. 
Herbs, however, are not entirely lacking as fossils. In the middle 
Cretaceous of the Potomac occurs an impression which has been variously 
assigned to Plantago , the Xyridaceae, and the Cyperaceae, and which is 
obviously herbaceous. In the Florissant, presumably Eocene, occurs a leaf 
referred to Car dims. From the Miocene onward, especially in the strata 
from Spitzbergen and Switzerland investigated by Heer (9 and 10), herba- 
ceous forms are increasingly abundant. In the Tertiary flora of Switzerland 
Heer records 24 per cent, of the Angiosperms as being herbaceous. In the 
strata which he studied, the number of herbs is much greater in the more 
recent than in the older formations. 
On the whole, therefore, evidence from palaeobotany as to the relative 
antiquity of herbs and woody plants cannot be called conclusive. It seems 
to indicate that the earlier Angiosperms were mainly trees and shrubs, and 
that the number of herbaceous forms has gradually increased. The ease or 
difficulty of preservation, however, plays too important a part to make such 
evidence very convincing. It seems strange, nevertheless, that so very few 
herbs are found from the Cretaceous, and so many, comparatively, from the 
Tertiary, for conditions which would affect preservation do not seem to have 
been radically different in the two periods. In general, the evidence from 
palaeobotany, as far as it goes, seems to favour the theory that woody plants 
are more ancient than herbs. 
II. Evidence from Anatomy. 
A comparison between the stem structure of woody plants and herbs 
provides us with evidence which has a much more direct bearing on the 
problem of the relative antiquity of the two groups, and anatomical facts 
have consequently been the main basis for such inferences and conclusions 
as have heretofore been drawn. 
