5.3 Class Magnoliopsida – flowering plants - Cambridge University ...

238 ORDERING THE PATHS OF DIVERSITY
Figure 5.83. Trochodendron.
Figure 5.84. Gunnera.
Figure 5.85. Polygonaceae:
Reynoutria.
Figure 5.86. Amaranthaceae:
Ptilotus.
trochodendrales
This order has only two species of evergreen trees from east and South
East Asia each in its own family, Tetracentron sinense and Trochodendron
aralioides (Figure 5.83).
gunnerales
There are only two genera in the order. Gunnera has the familiar, massive,
palmate and deeply ribbed leaves. Usually grown beside water it
is also a colonist of land-slips. One advantage it has is the fixed nitrogen
it gets from the symbiotic blue-green bacteria (Nostoc) that live
in its exposed roots and rhizomes. It produces large strobiloid inflorescences
of tiny flowers, either bisexual or unisexual. Myrothamnus,
from tropical Africa and Madagscar, is a resurrection plant, appearing
to dry out but able to revive and start growing again when water
becomes available.
Core Eudicots
Core eudicots have predominantly flowers with parts in fives (pentamerous)
with a clear distinction between calyx and corolla. There
are two main lineages, the Rosids and Asterids, three large basal lineages,
the Caryophyllales, the Santalales and the Saxifragales, and a
number of others of uncertain relationship like the Berberidopsidales
and Vitales. These basal orders are crassinucellate (see below).
caryophyllales (catchflies, stonecrops and cacti)
The Caryophyllales is a large, and an interesting group of about 4%
of all flowering plants that exhibits a unique set of characters (see
Figure 5.86). For example it seems to lack mycorrhizae. It comprises
mostly herbs, but others are lianes or twiners and shrubs. They have a
peculiar pattern of secondary growth with the production of diffuse
or successive cambia, which is commonly associated with succulence
and CAM photosynthesis. Many families that have a high frequency
of succulence have species that are either xerophytic like the cacti
(Cactaceae), stonecrops (Aizoaceae) or halophytic like the sea-lavenders
and thrifts (Plumbaginaceae).
Most have a campylotropous ovule in which the inner integument
protrudes, and a peripheral embryo surrounding a nutritive central
perisperm tissue and so they were previously called the Centrospermae.
Another shared character is a peculiar type of sieve-tube plastid,
and their chemistry is distinct. The core caryophyllid families are
Amaranthaceae, Aizoaceae, Cactaceae, Caryophyllaceae, Didiereaceae,
Molluginaceae, Nyctaginaceae, and Phytolaccaceae. Most of these have
a shikimic acid biosynthetic pathway as a starting point for the synthesis
of nitrogen-containing benzylisoquinoline alkaloids and the
betalain pigments. The latter are utilised instead of the anthocyanins
used in other flowering plants.
Non-core caryophyllids are the Plumbaginaceae, Polygonaceae,
Tamaricaceae, and Frankeniaceae in one clade and four families
of insectivorous plants in a sister clade. The former include many
halophytic plants, which are also found in core caryophyllids such
as the Amaranthaceae and Chenopodiaceae (Salicornia). Many have