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of the University of California at Los 
Angeles we may at least know some- 
thing about the environment in which 
the angiosperms evolved and why their 
appearance was so sudden. His hypoth- 
esis is that the angiosperms arose in 
arid and montainous regions where the 
ponds and lakes that figure so promi- 
nently in the formation of fossils would 
certainly be rare. Without muddy lake 
bottoms, which are free of decay-caus- 
ing oxygen and rich in fine-textured 
sediments that preserve the form and 
even the tissues of delicate structures, 
few fossils would be formed. There 
would be exceptions, of course, and 
there are some large lakes in mountain- 
ous regions. These lakes, however, 
would never be filled with the silts and 
clays that often protect fossil deposits 
in lowland lakes. Thus when mountain- 
ous regions are eroded away, the high- 
elevation lakes and fossils will go with 
them, and any traces of life they carried 
will be destroyed. If the Axelrod hy- 
pothesis is correct, the first steps in the 
evolution of the angiosperms may be 
lost forever. 
The hypothesis also suggests some- 
thing about the population structure of 
the ancestral angiosperms. Mountain- 
ous terrain is likely to be heteroge- 
neous, and if the regions inhabited by 
the early flowering plants were arid as 
well, the plants growing there would 
probably have been limited to particu- 
larly favorable sites. Perhaps the first 
angiosperm populations occupied only 
the less arid swales and ravines and 
were separated from one another by 
hummocks and ridges. This population 
structure — a series of medium-sized 
groups separated by short distances — 
happens to be the one thought to best 
provide two requirements of rapid evo- 
lution: a good supply of genetic vari- 
ation and a measure of isolation. The 
genetic variation contained in medium- 
sized groups may be adequate to pro- 
duce individuals adapted to local 
conditions, but even if it is not, an occa- 
sional seed or pollen grain from a near- 
by group could replenish the stockpile. 
Moderate isolation would allow this oc- 
casional input of genetic material while 
at the same time preventing a flood of 
material that would disrupt adaptive 
gene combinations already present 
within the population. 
The Axelrod hypothesis made a ma- 
jor contribution to the understanding 
of early angiosperm history, but unfor- 
tunately it tells little about how these 
newly evolved plants were able to out- 
compete and displace their gymnosper- 
mous antecedents. Here we must look 
to the angiosperms themselves. Their 
competitive superiority has been attrib- 
uted to many different characteristics. 
For example, angiosperms generally 
produce broader leaves than gymno- 
sperms; this enables the flowering 
plants to intercept more light for photo- 
synthesis. Angiosperms also produce 
an amazing arsenal of foul-tasting or 
even toxic substances that presumably 
protect them from grazing animals and 
insects. Furthermore, angiosperms, un- 
like nearly all gymnosperms, possess 
vessel elements, specialized conducting 
cells that form highly efficient vascular 
systems. 
Each of these characteristics has been 
implicated in the adaptive success of the 
angiosperms, but none is limited to the 
angiosperms. Ginkgo, a gymnosperm, 
produces broad leaves; the resins of the 
conifers clearly discourage even the 
most voracious herbivores; and Gne- 
tum, another gymnosperm, produces 
vessel elements, as does bracken fern. 
Moreover, the more primitive angio- 
sperms apparently lacked vessel ele- 
ments. Perhaps all these adaptive 
features are more the products of angio- 
sperm success than its cause. At best, 
they are probably only a part of the an- 
swer. For the rest, let us consider an- 
other characteristic of the angiosperms: 
their mode of pollen transport. 
The primitive angiosperms were pol- 
linated by insects, as are most of the 
present-day angiosperms. (There have 
been some reversions to wind pollina- 
tion, but these occurred after the essen- 
tial features of the angiosperms had 
evolved.) An intuitive presumption 
that insects are more reliable than the 
wind as a means of pollen transport is 
supported by some excellent studies by 
Robert W. Cruden of the University of 
Iowa. He has shown that with insect- 
pollinated species 6,000 pollen grains 
may be produced for each ovule, a large 
ratio but one that pales in comparison 
with the million-to-one ratio found in 
wind-pollinated species. This disparity 
suggests that even if insects consume a 
great quantity of pollen (up to 90 per- 
cent according to some estimates), they 
are still less profligate than the wind. 
In addition, because insects fly more 
or less directly from flower to flower, 
they are far more likely than the wind 
to accomplish pollen transport between 
such moderately isolated subpopula- 
tions as the groups of early angio- 
sperms proposed by Axelrod. The wind 
can carry pollen for many miles, occa- 
sionally covering the decks of ships far 
32 
