Taxonomy of Flowering Plants - LECTURE NOTES
Hugh D. Wilson
 

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. (overview)

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. (overview)

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 (overview) 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: (overview)
 
 

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

Return to Lecture Notes, the Botany 301 homepage, the next lecture, or the prior lecture