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Mesozoic Vegetation
A few words in the language of the modern botanist will show how
this vegetation harmonises with the story of evolution. Plants
are broadly divided into the lower kingdom of the Cryptogams
(spore-bearing) and the upper kingdom of the Phanerogams
(seed-bearing). As we saw, the Primary Era was predominantly the
age of Cryptogams; the later periods witness the rise and
supremacy of the Phanerogams. But these in turn are broadly
divided into a less advanced group, the Gymnosperms, and a more
advanced group, the Angiosperms or flowering plants. And, just as
the Primary Era is the age of Cryptogams, the Secondary is the
age of Gymnosperms, and the Tertiary (and present) is the age of
Angiosperms. Of about 180,000 species of plants in nature to-day
more than 100,000 are Angiosperms; yet up to the end of the
Jurassic not a single true Angiosperm is found in the geological
record.
This is a broad manifestation of evolution, but it is not quite
an accurate statement, and its inexactness still more strongly
confirms the theory of evolution. Though the Primary Era was
predominantly the age of Cryptogams, we saw that a very large
number of seed-bearing plants, with very mixed characters,
appeared before its close. It thus prepares the way for the
cycads and conifers and ginkgoes of the Mesozoic, which we may
conceive as evolved from one or other branch of the mixed
Carboniferous vegetation. We next find that the Mesozoic is by no
means purely an age of Gymnosperms. I do not mean merely that the
Angiosperms appear in force before its close, and were probably
evolved much earlier. The fact is that the Gymnosperms of the
Mesozoic are often of a curiously mixed character, and well
illustrate the transition to the Angiosperms, though they may not
be their actual ancestors. This will be clearer if we glance in
succession at the various types of plant which adorned and
enriched the Jurassic world.
The European or American landscape--indeed, the aspect of the
earth generally, for there are no pronounced zones of climate--is
still utterly different from any that we know to-day. No grass
carpets the plains; none of the flowers or trees with which we
are familiar, except conifers, are found in any region. Ferns
grow in great abundance, and have now reached many of the forms
with which we are acquainted. Thickets of bracken spread over the
plains; clumps of Royal ferns and Hartstongues spring up in
moister parts. The trees are conifers, cycads, and trees akin to
the ginkgo, or Maidenhair Tree, of modern Japan. Cypresses, yews,
firs, and araucarias (the Monkey Puzzle group) grow everywhere,
though the species are more primitive than those of today. The
broad, fan-like leaves and plum-like fruit of the ginkgoales, of
which the temple-gardens of Japan have religiously preserved a
solitary descendant, are found in the most distant regions. But
the most frequent and characteristic tree of the Jurassic
landscape is the cycad.
The cycads--the botanist would say Cycadophyta or Cycadales, to
mark them off from the cycads of modern times--formed a third of
the whole Jurassic vegetation, while to-day they number only
about a hundred species in 180,000, and are confined to warm
latitudes. All over the earth, from the Arctic to the Antarctic,
their palm-like foliage showered from the top of their generally
short stems in the Jurassic. But the most interesting point about
them is that a very large branch of them (the Bennettiteae) went
far beyond the modern Gymnosperm in their flowers and fruit, and
approached the Angiosperms. Their fructifications "rivalled the
largest flowers of the present day in structure and modelling"
(Scott), and possibly already gave spots of sober colour to the
monotonous primitive landscape. On the other hand, they
approached the ferns so much more closely than modern cycads do
that it is often impossible to say whether Jurassic remains must
be classed as ferns or cycads.
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