desert calendar - Desert Magazine of the Southwest

Fig. 3—Early Devonian landscape with first land plants.
From left to right: Rhynia, Psilophyton, Baragwanathia,
Asteroxylon, and Hornea. These were all
small, less than two feet high, reproduction was by
way of spores, all bore stamp of their marine ancestors.
They lived in marshy inlets near the sea. Climate
was warm all over the earth. About 375 million
years ago.
lows and poplars, where some trees
are male and some female. Some of
the extinct types called Cordaites modified
this system so that pollen and
ovules were produced on the same
plant but on separate branches, a system
still popular with the oaks and
alders. It is only a short step from this
scheme to one found among members
of the spurge family, castor beans for
instance, where both male and female
flowers occur close together on the
same shoot with the male flowers at
Fig. 5—A Carboniferous swamp. Left to right: Calamites
or horsetail rushes of tree-size grow on low
ground, fern-like plant is Lyginopteris, trees with
tufts like gigantic bottle brushes are Sigillaria. Large
tree at right foreground with rows of scars is Lepidodendron.
Tree with smooth trunk and yucca-like
leaves is a Cordaite. Fronds of an early fern hang
into picture at lower right. Climate seems to have
been mild to warm and moist. 270 million years ago.
the bottom and the female flowers at
the top. The next step is still shorter
and results in the so-called perfect
flower. This can be a very simple setup
with bare stamens and pistils close
together like the flowers of Yerba
Mansa.
The rough idea of the modern
flowering plant seed was illustrated by
the seed-ferns. These were plants with
fern-like habits but which reproduced
by means of seeds. This was far back
in the Carboniferous period, around
Fig. 4—Later Devonian plants. From left to right: Alethopteris,
Calamophyton, young frond of a seed-fern,
Eospermatopteris an early seed-fern the size of a
small tree. Archaeosigillaria a giant clubmoss grows
on land spit in middle distance, the trunk with scars
at right is the same tree after it has shed its lower
leaf-scales. Climate appears to have been warm and
semi-arid. 345 million years ago.
360 million years ago. These seeds,
about the size of hazel nuts, grew at
the ends of special branches. Since
seeds are developed from ovules this
means that ovules also grew at the
branch tips. Now, some later species
of seed-ferns show a modification of
the branch tip with the ovules partly
enveloped by a cup-shaped outgrowth
from the stalk, this is called a cupule.
Still later, some millions of years later,
other plants improved upon the cupule
so that it enclosed the ovules entirely
Fig. 6—Scene in Texas during the Permian period. Coniferous
trees are Ullmannia with cones at left,
Auricarioxylon in foreground; other conifers related
to star-pines and monkey-puzzle grow in a thin stand
on the bare soil in the middle distance, small plants
are Sphenophyllum and ferns. The lizard-like reptile
with crest is Dimetrodon, beyond are Diasparactus
zenos and Limnoscelis paludis. The background
of bad-lands and dust-filled sky were typical of the
period. Climate was dry-temperate to cold. 225
million years ago.
18 DESERT MAGAZINE