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;;


 I. The typical flower
    In the previous lecture we described a "typical" flower. However, few plants have "typical" flowers. In fact, finding a flower that looks like the "textbook diagram" is difficult. Most flowers are modified, to a greater or lesser extent, from the basic pattern.

    The fact that all flowers share the same basic floral plan is good evidence for evolution. If flowering plants didn't share a common ancestry, then there is no reason to expect that flowers would have the same plan. If each flowering plant was created separately, by individual acts of creation, then we would expect many different floral designs.

    Stephen J. Gould describes the flowers of some orchids as imperfections of nature. What he means is that the unique perianth structures found in these species are modified ("jury-rigged") from pre-existing structures (petals and sepals), rather than being built from scratch with the "best" or most "perfect" design for solving the problem of pollination. However, orchids (and all organisms) have no other choice - they are constrained by their evolutionary history. Thus, these "imperfections" show the tracks of evolution. Check out Gould's essay, "The Panda's Thumb".

II. Modifications
    Flowers can be modified in various ways including: (a) reduction of parts; (b) loss of parts; (c) fusion of parts; (d) displacement of ovary; (e) changes in floral symmetry; (f) changes in floral shape; and (g) floral morphs.

A. Reduction/Loss of Parts

Complete - all four floral organs present. incomplete - one or more absent. Perianth absent - naked; petals absent - apetalous. If there is only one whorl of perianth parts they are considered sepals. The sepals in apetalous flowers are often petaloid.

Perfect - both male and female present (regardless if sterile). The flower is considered bisexual. If one or other is missing, the flower is imperfect or unisexual.   staminate - unisexual male flowers, no carpels; pistillate - unisexual female flower, no stamens.

The terms synoecious (or hermaphroditic), monoecious (unisexual male and female flowers on the same individual such as in oak and birch) and dioecious (unisexual male and female flowers on different individuals such as in willow, ginkgo, hemp, ash, green ash, dates, and poplars) refer to how flowers are distributed on different plants. In some cases, both perfect and imperfect flowers occur on the same individual. This is termed polygamous. Polygamo-monoecious - with perfect, pistillate and staminate flowers on the same plant. Polygamo-dioecious have some individuals with perfect and staminate flowers, other individuals have perfect and pistillate flowers. Gynodioecious - plants with some individuals with perfect flowers and other individuals with female flowers. Gynomonoecious - individuals with perfect and female flowers. (andro- same but for male).

B. Reduction in number of parts in a whorl
    For example, the number of sepals in the poppy family (Papaveraceae) is reduced to two.

C. Fusion of parts

Connate - fusion of likes, such as the fusion of the petals (sympetalous or gamopetalous) or sepals (synsepalous). If the parts of a whorl are not attached and separate from one another they are considered to be distinct . Distinct petals are also termed polypetalous or choripetalous. 'Apo' is a prefix used to refer to parts that are distinct; 'Syn' or 'sym' refers to parts that are fused.

adnate - fusion of unlike parts such as the attachment of the stamens to the petals (epipetalous). free - different parts are separate.

D. Ovary Displacement or Floral Part Insertion
    This refers to the location of the ovary relative to the other floral parts. hypogynous - parts are inserted below (hypo) the ovary. The ovary is said to be superior. epigynous - parts inserted above (epi) the ovary. The ovary is inferior. perigynous - sepals, petals and stamens are united at the base and form a cup called a hypanthium; the gynoecium sits inside the cup. Peri means "about" - because the parts are attached "about" or around the ovary. The ovary is still considered superior.

E. Floral Symmetry.  
     Many flowers are radially symmetric or actinomorphic (regular).  This means that if you cut a flower in half anywhere through the center, you will have a mirror image. As an example, think of a bicycle wheel. Other flowers are bilaterally symmetric - zygomorphic (irregular), meaning you can only cut them in one plane and obtain a mirror image.

F. Floral Shape
    Flowers obviously vary greatly in shape and, naturally, there is a series of terms to describe floral shape. For example, a flower can be rotate (flat) or tubular (totally man!) or campanulate (bell-shaped) or funnel-form.

G. Floral Morphs
    In some species, individuals show different floral morphs called pin and thrum flowers. Pin flowers have a long style and short stamens whereas thrum flowers have a short style and long stamens. In these species, pollen from a short stamen is required to pollinate the short style and vice versa. This is an excellent mechanism for avoiding self pollination. In the water hyacinth, Darwin observed that there were two floral morphs: (1) long style, medium and short stamens; and (2) medium style, long and short stamens. Knowing that pollen from long stamens is required to pollinate long styles, and so on, Darwin predicted the existence of a third morph, short style, long and medium stamens. Interestingly, it was finally discovered in 1974 in South America after exhaustive searches (Barrett, 1989).

III. Floral Formulas
     Information concerning floral structure can be summarized using floral formulas. We will go over floral formulas in class and use a modified version of the one in our text. You should be able to read and write a floral formula. However, never memorize a floral formula. They should only be used to summarize information about a flower and as a study aid.  Floral formula handout.

IV. Floral Diagrams
     Again, another useful way of storing information about flower structure. Don't memorize these, but use them as study aids. You can even create your own floral diagrams. 

V. Floral Attachment - Inflorescences
   Flowers may be borne on a plant singly (solitary) or in clusters (inflorescence). The central axis is the rachis. Bracts, which are modified leaves, are often be found beneath a flower or inflorescence. The primitive condition is solitary and found in plants such as Magnolia, Winteraceae, and Calycanthaceae.

A. Pedicel vs. peduncle. Pedicel is the stalk that supports an individual flower. Peduncle supports an inflorescence.

B. Inflorescence Classification. They may be classified based on position and/or sequence of maturation.

  1. Position. Flowers may be terminal, axillary or intercalary (somewhere in middle of stem as in the Bottlebrush plant).
  2. Sequence of Maturation. 
        Flowers exhibit various maturation sequences. Determinate (cymose) inflorescences - 1st flower to open is terminal and the remainder proceed to mature downward, or from inside outward. Indeterminate (racemose) - 1st flower to open is basal, and matures upward or from outside inward. Note - in determinate inflorescences, once flower maturation is initiated, primary growth is finished. To complicate matters, some plants may have inflorescences that exhibit a mixture of the two types.

Determinate Inflorescence include:

Indeterminate Inflorescences include: 


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Last updated:  09/29/2008 / � Copyright by SG Saupe