Flowering Plants - Origin and Classification
The flora of a true "Jurassic Park" would contain no flowering
plants. Given the ubiquitous nature and aggressive tendencies of
modern angiosperms, it would be difficult to keep flowering plants out of
a large parkland today. However, during the Jurassic Period (180-135
million years ago), the global ecosystem - dominated by large reptiles -
was fueled by non-flowering vascular plants (mostly spore producers [ferns
and related types - the Pteridophytes] and gymnosperms).
Evidence of angiosperms appears in the fossil record near the beginning of
the Cretaceous Period, about 125 million years ago and this is in the form
of pollen. Archaic dicots and most monocots produce a relatively
simple type of pollen - monosulcate - that has only a single,
elongate pore through which the microgametophye emerges. This can be
distinguished from the pollen of gymnosperms and this is the type this is
rarely encountered in early Cretaceous sediments. However, as
indicated by expansion and differentiation of angiospermous pollen,
it appears that the flowering plants came to dominate the world's flora
with - from a geologic perspective - remarkable speed.
Elements referable to the dicot subclass Hamamelidae and Magnolidae
were evident by ca. 122 million years ago and, by the end of the
Cretaceous (ca. 100 million years ago) representatives of most of the
major extant flowering plant subclasses, both monocot and dicot, were
present.
As vascular plants, the Magnoliophyta are linked to the ferns and gymnosperms and, as seed-producing plants, they show a closer alliance to the gymnosperms. Given the structural variation among both extant and - especially - extinct elements of both types, it is difficult to identify the ancestral lineage for flowering plants. However, there is little doubt that the flowering plants are monophyletic, i.e., all extant types evolved from the same ancestral form and all share a common ancestor. This is because all flowering plants share a complex suite of features, especially those associated with the reproductive system, that would be difficult to acquire independently. The features represent flowering plant key characters.
Flowering Plants -
Key Characters:
Wood of most
gymnosperms (above) shows a relatively uniform texture in comparison to
angiosperm wood (below) because the secondary xylem is dominated by a
single type of cell (tracheids). Gymnosperm wood usually
lacks the large-diameter conducting cells (vessel elements) present
in the conductive vascular tissue tissue of most angiosperms.
Angiosperm phloem also differs from that of gymnsoperms by the presence of
more specialized and 'enhanced' conductive cells, sieve tube elements
and their associated companion cells.
All structures and functions associated with the carpel are unique to flowering plants. Pollination of gymnosperms involves direct contact with the exposed ovule and movement of the gymnosperm microgametophyte through an opening in integuments of the ovule, the micropyle, to the egg. The carpel, unique to flowering plants, forces a more complex path for the angiosperm microgametophyte. In addition, angiosperm double fertilization produces a triploid nutritive tissue - the endosperm - this is also unique to flowering plants.
The Magnoliophyta -
Classification:
Classification is fundamental in biology. Systems of classification are conceptual structures that reflect interpretation of available data regarding evolutionary relationships. Interpretations differ and new information is constantly emerging from many sources. Thus, flowering plant classification reflects an on-going scientific activity - there is no single, stable classification.
Efforts to classify flowering plants is, perhaps, the most ancient activity of Science. Pre-Darwinian classification systems either followed a metaphysical rationale, charting the Divine pattern of creation, or grouped flowering plants by abritrary ('artifical') criteria. Systems produced over the past 100 years have focused on the determination of phylogenetic or evolutionary patterns. This approach requires creation of a 'model' archaic or primitive type as a foundation from which modern groups evolved. Most current classification systems follow the model originally produced by Charles Bessey in 1915 and statements relating to 'phyletic polarity' (primitive vs. derived or specialized) presented in this course reflect, in most part, a 'Besseyan' perspective. Regardless of perspective, all current systems treat the flowering plants as a two-parted entity composed of the dicots (Magnoliopsida) and monocots (Liliopsida). Both types share the complex suite of features - described above - that circumscribe the flowering plants and all data available indicate that these features were present in an ancestral, dicot-like plant. Thus, the flowering plants appear to be monophyletic (both monocots and dicots evolved from a common ancestor) and the extant angiosperms most similar to the ancestral type are those placed in the basal dicot groups.
Since the two elements of
the Magnoliophyta diverged early in the evolutionary history of the group,
they are fairly distinct and, in terms flowering plant identification,
they represent the the first level of recognition. Key characters
used to circumscribe the two classes include:
| Character | Magnoliopsida | Liliopsida |
| Cotyledons | Usually 2 | Usually 1 |
| Leaves | Often reticulate veined | Often parallel veined |
| Vascular Cambium | Often present (ca. 50% woody) | Absent |
| Primary Vascular Bundles | Arranged in a ring | Often scattered |
| Pollen | Various types | Monosulcate |
| Floral Parts | Often in 4s or 5s | Usually sets of 3 |
| Root System | Primary and adventitious | Adventitious only |
It should be noted that all taxonomic circumscriptions are based on common or 'typical' key characters. Familiarity with patterns of variation expressed by this critical subset allows the student to gain an initial, functional overview and thereby develop the ability to place most unknowns within a particular group, category, or taxon. Once this position is established, it will always be necessary to incorporate, on a case by case basis, exceptional or atypical elements as they are encountered. This process often requires the student to evaluate the set of features presented by an unknown from a prioritized or weighted perspective. If, for instance, you encounter a shrub with net-veined leaves, and a 3-merous (parted) perianth, you must reject the perianth as a useful feature and conclude, on the basis of woodyness and leaf morphology, that it is a dicot. Critical evaluation of multiple key characters is especially important at the higher taxonomic ranks because, given the nature of comlex systems, there are always exceptions.
While all current systems
of angiosperm classification share a fundamental bifurcation of two
lineages, all differ with regard to organization with the dicot and
monocot groups. The Cronquist System, followed by your text, is the
least complex.
Magnoliopsida
| Subclass | Features | Orders | Families | Species |
| Magnoliidae | Archaic, many-parted flowers | 8 | 39 | 11,000 |
| Hamamelidae | Ancient, with floral reduction (wind pollination) | 11 | 24 | 3,400 |
| Caryophyllidae | Herbs with betalains and 'centered' placentation types | 3 | 14 | 11,000 |
| Dillenidae | Some sympetaly, little apocarpy | 13 | 78 | 25,000 |
| Rosidae | Polypetaly, often numerous stamens | 18 | 114 | 58,000 |
| Asteridae | Mostly sympetalous | 11 | 49 | 56,000 |
Liliopsida
| Subclass | Features | Orders | Families | Species |
| Alismatidae | Aquatics with archaic, many-parted flowers | 4 | 16 | 500 |
| Arecidae | Specialized inflorescences | 4 | 5 | 5,600 |
| Commelinidae | Mostly herbs with reduced flowers | 6 | 16 | 16,200 |
| Zingiberidae | Epigynous herbs of the tropics | 2 | 9 | 3,800 |
| Liliidae | Showy with petaloid sepals | 2 | 19 | 25,000 |
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