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

240 ORDERING THE PATHS OF DIVERSITY
Figure 5.91. Geraniales:
Geranium.
Rosids and Asterids
The remaining eudicots form two great lineages, the crassinucellate
(thick nucellus) Rosids and the tenuinucellate Asterids, each
split into two sister lineages (Eurosid 1 and 2, Asterid 1 and 2).
The tenuinucellate condition is a derived feature of the ovule where
the tissue layer, the nucellus, surrounding the developing megasporangium/embryosac
is thin. There are various kinds of crassinucellate
condition. Each main and sub-lineage is very variable but exhibits
particular evolutionary tendencies. For example, all the nitrogenfixing
families are found in the Eurosid I group: Casuarinaceae,
Myricaceae (Fagales), Eleagnaceae, Rhamnaceae, Ulmaceae (Rosales),
Fabaceae (Fabales) and Coriariaceae (Cucurbitales). There are well
defined families like the Fabaceae, almost stereotypical with its characteristic
fruit, the legume, and characteristic forms of flowers and
inflorescence, or large and diverse families, like the Rosaceae, that
have proved evolutionarily flexible.
5.3.7 Rosids
Rosid orders or even families are very diverse in their floral structure.
Most of the ecologically (and economically) important trees from
forests and savannas around the world are Rosids. They include the
tallest flowering plants such as the eucalypts, the mahoganies in the
tropics, the savanna acacias, and oaks, maples and beech from temperate
forests.
Percentage of families Rosids Asterids
stipules 66% with 16% with
stamens 58% with two whorls 79% single whorl
corolla 97% free 75% fused
integument(s) 94% two 87% one
nucellus 92% crassinucellate 91% tenuinucellate
endosperm 96% nuclear 75% cellular
iridoids 5% present 61% present
Basal Rosids
geraniales (geraniums)
The Geraniales include one large family, the Geraniaceae, and several
very small ones. One interesting feature they share is the presence
of glands on the margin of the leaf. They have pentamerous
obdiplostemonous flowers with a persistent calyx. The two largest
genera Pelargonium and Geranium produce similar beaked fruits but
differ in the former having monosymmetric flowers and a nectariferous
pedicel (Figure 5.91). The Geraniaceae are commonly herbs with
jointed stems but the other families in the order include shrubs and
trees.