tree-logo.gif (7741 bytes) Plant Taxonomy (BIOL308)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Overview of Vascular Plant Taxonomy

I.  Embryophytes – Land Plants

A.  Characters

  1. cuticle

  2. resting embryo in sporophyte

  3. multicellular sporophyte

  4. multicellular reproductive structure

  5. thick wall spores

 B.  Classification - traditionally two groups

  1. Bryophytes (mosses, liverworts, hornworts)

  2. Tracheophytes (vascular plants) – ferns, quillworts, horsetails, gymnosperms, angiosperms

C.  Cladogram (on board)
 

II.  Vascular Plants (Tracheophytes)

A.  Characteristics
    These are plants that possess vascular (transport) tissue; i.e., xylem and phloem. Other characteristics of the group include branched sporophytes, dominant sporophyte phase of the life cycle, and lignified walls. (Note: Although these are called vascular plants, other plant groups, specifically the mosses, have some vascular tissue, though it's not as well developed).  The vascular plants evolved about 420 million years ago (mya). The angiosperms arose later during the Jurassic (ca. 120 mya).

B.  Classification
    Cladistic evidence suggests that there are six major groups of tracheophytes.  These are:

    These groups (with the exception of the quillworts) are considered to be divisions of vascular plants by Cronquist (Polypodiophyta - the ferns; Psilotophyta - whisk ferns; Equisetophyta - horsetails; Lycopodiophyta - club mosses and quillworts);  Pinophyta (or gymnosperms) – "cone" bearing plants; and Magnoliophyta (or angiosperms) - flowering plants.  note the standard ending for division, -ophyta.).  

    Cladistic analyses essentially support these groupings. The main exception is that the Pinophyta, which has always been recognized as a diverse, polyphyletic assemblage, is separated into four groups - cycads, conifers, ginkgos, and gnetophytes - yielding nine major groups of vascular plants.

    The angiosperm and gymnosperm (including cycads, gnetophytes, conifers and ginkgos) groups (clade) are characterized by producing and dispersing by seeds, whereas the remainder disperse only via spores. (Quick review - both spores and seeds are reproductive units. The major differences are that spores are generally smaller than seeds and lack a pre-formed embryo.)

C.  Cladistic primer - we briefly discussed and defined monophyletic, paraphyletic, polyphyletic, node, clade, sister group


III.  Seed Plants

A.  Characters

B.  Groups

IV.  Angiosperms

A.  Characters

B.  Classification

1.  Traditional

Table 1: Comparison of features of Traditional Monocots and Dicots

Feature

Dicot

Monocot

Number of species 165,000 50,000
Growth form woody or herbaceous mostly herbaceous
Embryo cotyledons two cotyledon one (best character to distinguish)
Endosperm present or absent often present
Floral parts 4 or 5-merous 3-merous
Pollen grains tricolpate usually monocolpate
Leaves net (reticulate) veined (some exceptions like Plantago) parallel veined (some exceptions like Trillium)
Petiole common, seldom sheaths stem not common, petiole often sheathing
Vascular system definite # bundles, ring (like broccoli) numerous, scattered (like asparagus)
Secondary growth typically present (cambium) absent
Mature root system primary or adventitious, strong taproot often present wholly adventitious (primary root system of short duration, fibrous roots common, usually without strong taproot)
Cell walls glucan xylan
Gynoecium apo- or syncarpous mostly apocarpous

2.  Recent/Cladistic Views

  1. Eudicots – those that show traditional dicot features, this is a distinct monophyletic group

  2. Magnoliids – woody trees, show primitive features.  Includes families such as the Magnoliaceae

  3. Basal families – most primitive families 

  • recent work shows sister group to rest of angiosperms

  • no ethereal oils

  • elongate vessels

  • vessel less

  • carpels sealed by secretion

  • includes Amborellaceae, Nymphaeaceae, Illiciaceae

4.      Cladogram - check out the Angiosperm Phylogeny Website

C.  Evolutionary trends.  During our discussions we will mention evolutionary trends among the flowering plants. These ideas have been advanced largely by the "evolutionary classifiers" such as Cronquist, Thorne and their counterparts. The cladists might argue that these trends are recognized simply because they match the ideas of the classifiers (i.e., circular reasoning) and that true trends can only be deciphered by cladistic analyses. Nevertheless, these are a good starting point for studying the angiosperms.

V. The Importance of Families. We will focus on studying families for several reasons:

    1. Plants classified in the same family share a common ancestry and a set of recognizable characteristics. The trick is to learn the "theme" behind the variations.
    2. Learning family features is very useful because: (a) family taxonomy is relatively stable; (b) it is applicable to any flora; there are a manageable number to learn (about 400 families vs. 300,000 species of angiosperms); and (d) once learned, a given species is relatively easy to identify.
    3. There are many tips and tricks for learning family characteristics. My personal favorites - construct a mental "type" specimen from which you can reconstruct the features of the family, and preparing a comparison table. In fact, I’d start a comparison table now and fill it in as we add new families. Check Zomlefer’s book for some good examples.

References:

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Last updated:  08/20/2007 / © Copyright by SG Saupe