3
How Plants Are Identified
T
he easiest method, and the one probably
used the most often for learning the
names of the plants of a particular location
or region, or just the name of a particular plant, is
to ask someone who already knows. Perhaps the
way used to learn plant names most often is to
compare the unknown plant with the photograph
or sketch of a similar one in a book. Many picture
books have examples of the more common plants,
but most species cannot be identified in this fashion.
The third way is to identify the plants for oneself. To
have the skill to identify an unknown plant is a most
valuable asset and an important part of the study of
systematics.
Identification of unknown specimens is usually
made by using a key—that is, a device in which successive choices between contrasting statements are
followed until the correct name is found by the process of elimination. Keys play an integral part in a
flora, allowing for proper identification of families,
species, and infraspecific taxa.
14
Most modern keys are constructed of paired
choices. Each half of the paired choice makes a statement or several statements that are either true or false.
The “two forked,” “couplet,” or “paired choice” keys
are called dichotomous (meaning forking) keys.
The first of this type was developed by Jean-Baptiste
de Lamarck, the famous French botanist. In 1788, as
a shortcut method to reading down the long lists and
descriptions of plants found in the botanical literature
of the day, he provided an artificial key as a means to
identify the plants of France (Flore Française).
Using a dichotomous key is like following directions to a distant destination. As one travels down
a road or highway, decisions are made at junctions
as to which way to turn. If the roads are properly
marked, and if the directions are followed carefully,
the destination should be reached.
There are two main types of botanical keys: the
indented key and the bracketed key. Figures 3.1
and 3.2 illustrate how both of these types of keys
function for seven objects. In both keys, the objects
How Plants Are Identified
1. Object with curved sides
2. Sphere solid white in the middle
2. Sphere black or partially black
3. Sphere all black
3. Sphere one-half white and one-half black
1. Object with straight sides
4. Object with four equal sides
5. Square white
5. Square one-half black and one-half white
4. Object with three equal sides
6. Triangle with spines at the points
6. Triangle lacking spines
Some Basic Rules for Constructing
and Using Dichotomous Keys
a sphere
Some basic rules common to all types of keys should
be followed when constructing and using dichotomous keys:
a square
a triangle
Figure 3.1 An example of an indented key.
are divided into subgroups; by the process of elimination, the unknown can be identified. All objects must
be accounted for, with none left out.
In an indented key, the paired-couplet choices
are identified in the same way and given the same
number. This is important because, in large keys to
many species, the halves of the couplet may be separated by some distance from one another and may
even be on different pages. Some authors use letters
(i.e., a, b, c,…or aa, bb, cc, etc.) or symbols (i.e., 1,
1´, 2, 2´,…etc.) in place of numbers to keep from
confusing couplets.
The bracketed key follows the same basic principles of contrasting choices and choices given the
same number, letter, or symbol as an indented key.
The choices, however, are always placed on adjacent
lines, thus requiring less room on a page, and the
couplets are kept together. Relationships between
taxa are not as easily observed in bracket keys; and
when mistakes in keying are made, backtracking is
more difficult. Bracketed keys are commonly used in
beginning, student-type keys.
1. Object with curved sides; a sphere
1. Object with straight sides
2. Sphere solid white in the middle
2. Sphere with black
3. Sphere all black
3. Sphere one-half white and one-half black
4. Object with four equal sides; square
4. Object with three equal sides; triangle
5. Object white
5. Object one-half white and one-half black
6. Triangle with spines at the points
6. Triangle lacking spines
Figure 3.2 An example of a bracketed key.
15
2
4
3
5
6
1. Construct all parts of the key in a dichotomous
fashion. In certain disciplines of biology or in
older botanical manuals one will sometimes
encounter keys with many branches. This can
be very confusing and time-consuming and can
cause one to make incorrect choices.
2. Always use a contrasting alternative condition of
a plant character in the other half of the couplet.
This means that if the character is a leaf character, the alternating leaf character must also deal
with the same leaf feature. For example:
1. Leaves attached alternate
2. Leaves attached opposite
Using the alternating choice “leaves heartshaped” would not be a correct contrasting
choice, even though it is a choice dealing with
leaves.
3. Construct the leads parallel within each couplet. This means that the beginning word of
each alternating choice should be the same. For
example, if the word of the first lead is petals,
then the first word of the contrasting couplet
should also be petals.
4. Describe the characters of the plant in a positive manner. The person using the key should
be able to get a visual image of the condition of
each character used.
5. Avoid using vague, unclear, overlapping measurements and general, broad terms (e.g., large
vs. small, dark vs. light colored, 5.0–10.0 mm
[0.2–0.4 in.] vs. 8.0–12.0 mm [0.3–0.5 in.]
long).
6. Keep the season of the year during which the
key is to be used the same; that is, if the key
is to winter twig features, they should not be
mixed with the characters of leaves. An easy way
to accomplish this and to make the key more
seasonably usable is to include flower, fruit, and
vegetative characters together in the couplets of
the key.
7. Avoid using taxonomic names as key characters.
The key may, however, be designed to key out
16
Contemporary Plant Systematics
taxonomic categories above the rank of species,
as well as species below the species level.
8. Choose characters for a key that have the most
reliable, least variable features available but that
are still easily observable. Generally, flower features are less variable than vegetative ones.
9. Omit highly technical and obscure features as
key characters (e.g., chromosome morphology
or number, detailed anatomical features, the
presence or absence of compounds, and localities). The person using the key normally does
not go to great lengths to get the information
requested by a couplet.
10.Consider both sexes in species that are dioecious (reproductive structures on different
plants), because the specimen to be identified
may have flowers of only one sex.
11.Try to avoid difficult or irregular arrangements
or identification words in the couplets. The
purpose of the key is to provide an easy method
to identify an unknown plant specimen. Therefore, the design of the keys should be for convenient and self-evident use.
12.Remember, keys are not absolutely reliable
and without mistakes. The key is only as good
as the material used in its construction. Plant
specimens that exhibit extreme variation and
have not been observed by the writer of the
key may not key out properly. Also, interpretation of characters may vary, so the student
should understand the author’s use of a term.
It should be remembered that the key is another
person’s way of interpreting how a group of plants
should be identified. The clarity and ease of use of a
key depends on many factors; therefore, it should be
remembered that a key is not magic, and that to master a key may take some time before one can identify
unknown plants without making errors. Keying
specimens takes skill and considerable practice. One
should not guess when keying, if at all possible.
Guessing almost always brings a wrong ­determination.
Correctly learning and applying terminology is the
best way to identify and key plants.
Other Identification Methods
The keys we have been discussing provide the traditional tools that botanists use to identify plants. In
recent years, however, other techniques have developed that do not follow the very structured, singlebeginning point of the dichotomous key. These are
the polyclaves and computer methods.
The polyclave, or synoptical, keys are multientry keys that use cards stacked in any arrangement
on top of one another, with holes or edges punched
in such a way as to allow cards with the desired taxa
to be retained or eliminated until the card with the
desired taxon listed on it is the only card remaining.
The holes or punched edges correspond to characters
chosen by the investigator.
During the 1960s, computer technology methods and their associated programs developed greatly.
Programs began to be developed that would provide for automated identification of specimens by
the computer, computer-developed multi-entry keys,
and computer-stored information. The past thirty
years have seen the rapid development and use of the
micro­computer (MacLeod, 2007). These machines
and the programs perfected for them have expanded the potential for their use—especially for smaller,
more specific groups. As the technology and programs
are further perfected, future use of them will certainly increase. Accompanying this text is a multi-entry
computer key to the 519 families, including those illustrated in the Photo Atlas DVD that also accompanies it. By looking at the readily available features of
the live specimen in hand, following the instructions
given by the CD, and clicking on the choice that best
fits the features, the name of the family and photo
images of species examples found in the family can be
observed. This method will allow you, the student, to
identify the world’s most common vascular plant species to family using a personal computer.
Selected references for this chapter appear in Appendix I.
Families of Flowering Plants (Angiosperms)
173
Family Monimiaceae (Monimia)
Figure 10.11 Hedycarya: (a) leafy branch with male flowers in axillary clusters; (b) female flower with indi­
vidual pistils. Peumus: (c) front view of female flower; (d) pistil or gynoecium; (e) stamen with two basal staminodes;
(f) front view of male flower.
(b)
(c)
(a)
(e)
(f)
General Description: shrubs or woody vines; fragrant
ethereal oil cells and needlelike calcium oxalate crystals
usually present in the parenchyma tissues; aluminum
­accumulators.
Leaves: simple, opposite (rarely alternate), serrate,
punctuate dotted with special looped venation joined
near the margin; no stipules.
Flowers: small to medium size, regular, perfect or uni­
sexual, hypogynous or perigynous with a cup-shaped
hypanthium, commonly with nectar glands inside; inflo­
rescence of a solitary flower or flowers in axillary cymes.
Perianth: of 2 pairs of fleshy sepals. Petals: 7–many,
sometimes indistinguishable from one another or lacking
altogether. Stamens: many, filaments short, in 1–2 series,
with or without paired nectar glands at the base; anthers
opening by longitudinal slits or valves upward from the
base. Pistil: simple, of 1–many individual carpels; locule 1;
ovule 1 and born on an apical or basal placenta.
Fruit: of individual drupes or nuts enclosed by the
perianth.
(d)
Seed: small- to medium-sized embryo and oily en­
dosperm.
Distribution: 22 genera and 200 species in subtropical
and tropical regions mostly in the southern hemisphere.
The largest genera are Kibara (45 species), Mollinedia
(90 species), and Tambourissia (45 species) found in
the tropics.
Economic Value: little, except locally where aromatic
oils from the bark and leaves are used in perfume and
medicines (Peumus boldus).
Classification: cup-shaped, concave floral axis and car­
pels sunken into the cup tissue make the family distinc­
tive from other “magnolia-type” flowers.
Chromosome Numbers: n = (18-) 19 (-22, 39, 43,
57, etc.).
Fossil Record: some Eocene fossils and fossil pollen
from Oligocene and more recent deposits.
Families of Flowering Plants (Angiosperms)—Eudicots I
(a) Core Eudicots (1), Order
Caryophyllales, Family Caryophyl­
laceae, Lychnis chalcedonica “Rosea”;
face-view of corolla.
(d) Core Eudicots (1), Order
Caryo­phyllales, Family Droseraceae,
Dionaea muscipula; trap leaves show­
ing trigger hairs.
(g) Core Eudicots (1), Order
Caryo­phyllales, Family Nepenthaceae,
Nepenthes raja; large leaf pitcher.
299
(b) Core Eudicots (1), Order
Caryo­phyllales, Family Chenopodi­
aceae, Chenopodium album; flowers
close-up and scurfy surface feature.
(c) Core Eudicots (1), Order Caryo­
phyllales, Family Chenopodiaceae,
Salicornia virginica; clone habit.
(e) Core Eudicots (1), Order Caryo­
phyllales, Family Droseraceae Drosera
sp., undetermined species; close-up of
glandular hairs.
(f) Core Eudicots (1), Order Caryo­
phyllales,
Family
Molluginaceae,
Mollugo verticillata; fruits and whorled
leaves.
(h) Core Eudicots (1), Order
Caryophyllales, Family Nyctaginaceae,
Bougainvillea spectabilis, undetermined
species; colored bracts around flowers.
(i) Core Eudicots (1), Order Caryo­
phyllales, Family Phytolaccaceae,
Phytolacca americana; flowers and
young fruits.
PLATE 11.5 Flowering Plants (Angiosperms), Eudicots (Tricolpates), and Some Core Eudicots