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. Terminology
    Taxonomists need descriptive terminology to efficiently and accurately communicate information. This is particularly important because taxonomy is, in large part, a descriptive science (vs. experimental science). It is similar to anatomy and related disciplines in regard to the number of terms used.  Tyson (2001) has written a fun article that argues that some scientific disciplines, specifically astrophysics, uses less jargon/terminology than other sciences. 

 II. Why so much terminology
    The amount of terminology used in Plant Taxonomy may, at times, seem overwhelming.  The following description of 'how they hitch horses in Europe' from A Tramp Abroad by Mark Twain should hopefully explain why there is so much terminology.....

     The man stands up the horses on each side of the thing that projects from the front end of the wagon, throws the gear on top of the horses, and passes the thing that goes forward through a ring, and hauls it aft, and passes the other thing through the other ring and hauls it aft on the other side of the other horse, opposite to the first one, after crossing them and bringing the loose end back, and then buckles the other thing underneath the horse, and takes another thing and wraps it around the thing I spoke of before, and puts another thing over each horse's head, puts the iron thing in his mouth, and brings the ends of these things aft over his back, after buckling another one around under his neck, and hitching another thing on a thing that goes over his shoulders, and then takes the slack of the thing which I mentioned a while ago and fetches it aft and makes it fast to the thing that pulls the wagon, and hands the other things up to the driver.

III. Phytography
    A fancy term for studying the terminology of botany is phytography. Most terms are derived from Latin or Greek, the language used by scholars during the early history of taxonomy.

IV. Characters and States
    A feature or observable attribute of a plant is called a character and the specific form or expression of the character is called a state. For example, flower color is a character with several states including red, yellow, white and blue flowers. All characters are not equally important for identification purposes. The most useful features are called "diagnostic" or "key" characters. These are given more "weight" when making decisions about a particular taxon. For example, fruits are diagnostic characteristics in the Mustard family (Brassicaceae); without fruits, these plants are difficult to identify.

    Some characters are rather variable (e.g., leaf shape, stem height, time of flowering) while other characters are fixed (i.e., floral characters). The source of the variation observed in these characters can either be environmental or genetic. One way to distinguish between them is to do transplant experiments. 

V. Vegetative Terminology
    In this section of the course we are interested in terms related to the vegetative, or non-reproductive parts of the plants. This includes the three primary organs - leaf, root, stem. Later, we will discuss terms relating to the reproductive organs (flowers, fruits).

VI. Discussion of Selected Vegetative Terms
    We may go into the field (or even grocery store) to observe and discuss the following terms. The terms in the following list are "fair game".

A. Life span/Duration

It can be difficult to determine whether a herbaceous plant is an annual, perennial or biennial. Some tricks (adapted from Woodland, 1997) are to examine:

  1. the underground part of part of the plant - annuals have small slender taproots, while those of biennials and perennials are larger. And, biennials and perennials often have food storage structures such as tubers or bulbs
  2. whether the stem is woody or herbaceous - plants with herbaceous stems are often annuals, those with woody stems are perennials
  3. if there are any constrictions or scars where the stem joins the roots - annuals typically lack these while perennials have them
  4. the growth form - a plant with a rosette late in the growing season may be a biennial
  5. remnants of last year's leaves or stems - the presence of these indicate a perennial or biennial

B. Plant habit - refers to general growth form

    Trees and shrubs are woody perennials. Annuals are often herbaceous.

C. Habitat - refers to environment in which plant grows

D. Plant vegetative organs

    Some plants lack one or more of these structures.  For example, aquatic plants often lack roots and cacti lack leaves.  There is no clear anatomical distinction between leaf and stem. The crown is the junction of the root and stem.  A shoot is a stem with leaves

E. Root types

F. Stem types

G. Stem features

H. Leaf parts

I.  Leaf structure

It may be difficult to determine whether a plant has compound leaves or is a branch with simple leaves. Some tricks include:  

  1. make a cross section of the suspected petiole - if it's round it is likely to be the petiole, if not, then it is probably a petiolule of a compound leaf;
  2. check the branching pattern - if the branches are alternate, even though the "leaves" appear opposite, then the leaf is probably compound;
  3. check to see whether the "leaves" are in a single plane or not - leaflets of a compound leaf are in a single plane, but leaves on a branch tend to be in different planes;
  4. check the terminal leaf - the terminal leaf of a branch tends to be offset from the apex whereas the terminal leaf of a leaflet is truly terminal;
  5. look for a bud - only leaves have a bud in the axil;
  6. check for flowers - only leaves will have flowers in the axil;
  7. check for the color and texture of the leaves and stem - in compound leaves, the leaflets and rachis are usually similar, whereas the color and texture of the branch usually differs from the petiole and blade.

J. Leaf attachment & position

K. Arrangement
Leaves may be found only at the base of the plant (basal, rosette, as in dandelion) or along the stem (cauline) or some combination.  Leaves are attached at nodes:

L. Venation

M. Leaf margin

N. Special features

O. Surface features

P. Leaf shape
    There are many terms to describe the shape of the leaf blade (linear, oblong, lanceolate, elliptic, ovate, scale-like, needle-like), leaf apex (acute, obtuse, acuminate, mucronate, truncate, emarginate, rounded), and leaf base (acute, acuminate, obtuse, rounded, truncate, cordate, oblique, hastate, sagittate).

 VI. Monocot leaves
    They are usually simple, with parallel venation, linear and have a sheath which is considered to be the basal part of the petiole. In fact, there has been such a severe reduction in the blade that the flattened blade in many monocots is derived from the leaf base (petiole).

VII. Some general points to consider

  1. usage of terms may differ slightly from author to author;
  2. many characters show continuous variation, so the character state may fall midway between two terms. An analogy would be compass positions; if the needle points between N and E we call that direction northeast. But what if it falls between NE and N, and so on? One solution is to combine terms - for example, linear-lanceolate leaves would have an intermediate shape; 
  3. a single plant may show a range of features (i.e., it could have some lobed leaves and others that are entire);
  4. diminutives are often used (i.e., serrulate leaves would have smaller teeth than a serrate ones); and
  5. prefixes are often used to clarify meanings. For example, "sub" means "almost". Thus sub-opposite leaves are not quite opposite. And "ob" means opposite, such as in obovate leaf shape.

IX. References

X. Websites

XI.  Exercises & Study Hints

  1. For some study hints and lab exercises - click here
  2. Terms that are "fair game" for the exam are highlighted in this document.
  3. Check out the Texas A&M websites and Vascular Plant Systematics Glossary listed above.  They are terrific sources rich with images and some quizzes.

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