PLANT TAXONOMY (Identification, classification and description of plants)

TAXONOMY: THE STUDY OF IDENTIFICATION, 

CLASSIFICATION, AND NOMENCLATURE

A COMBINATION OF SCIENCE AND 

ART

Taxonomy or Plant Systematics, despite what people would have 

you believe, really is not an exacting science in many ways

•

This statement mainly applies to the identification process, so 
we’ll start there.

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Identification is very different from classification, which is 
even more problematic

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There are several methods for identifying plants

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Many books rely on matching a description or illustration with 
the plant you have in hand

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Most people first go to books with color photos, but actually 
good line drawings can show more detail

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Books with color photos or drawings often are arranged by 
color, but this is imprecise because of different color 
perceptions by different people, and some genera fall in many 
color categories, making finding the species difficult

The vast majority of horticulture books use the color or form 

method for identifying rather than discrete, consistent 

characters

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Besides Bailey’s 

Encyclopedia of Horticulture

and a few others, 

few horticulture books cover the whole spectrum of garden 
plants, leaving many possibilities out

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Few horticulture books address a 

Key

for correct identification 

(more about keys in a moment)
If you’re interested in a special group of garden plants like 
roses, chrysanthemums, and cacti there are books covering 
those subjects in fair detail, making i.d. somewhat more 
practical

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Currently, the best way to id garden plants is by learning to 
key to family and then consulting books on genera, if available

Identification of native plants, by contrast, is often a surer thing; 

many states have floras of their native plants

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Even better for the beginner are books specializing in one 
particular geographic area, such as Marin County

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The majority of these i.d. books contain not only some 
illustrations, but 

dichotomous keys

for making a 

determination

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The inexact part of this aspect of i.d. is because keys contain 
many inconsistencies and sometimes just plain mistakes

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The other inexact part is that plants vary a lot in the wild, and 
no keys take all of the variation into account

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For example, the genus 

Fragaria

(strawberry) has flowers with 

5 petals, but occasionally an individual will display 6 petals, 
instead. This could completely mislead the identifier because 
number of flower parts is heavily emphasized

Dichotomous keys also require knowledge of terminology, since 

many terms are more precise than using ordinary words

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With flowering plants, the starting point is usually 
determining if your plant is a 

monocot 

or 

dicot

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Those two major categories are based on several traits, but 
the terms themselves refer to the number of seedling 
leaves—two for dicots, one for monocots

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This trait is impractical to use in most cases, since plants lose 
their seedling leaves soon after germinating

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There are also exceptions to the number of cotyledons, as 
there also are for most criteria to recognize these two groups

Fortunately, there are other rules that help determine monocots 

and dicots, which are easier to apply

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The two most important traits are leaf vein pattern (veination) 
and numbers of petals and sepals

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Dicots usually have a network or featherlike pinnate pattern 
of veins while

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Monocots have the major veins parallel to each other

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However, there are occasional exceptions 

and

some leaves 

don’t show an obvious vein pattern

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For petal and sepal number, dicots have 4 or 5 (except for 
some early dicots that have a large number),

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Monocots have 3 or multiples of 3

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Again there are occasional exceptions

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There are other traits for the two groups but many, like pollen 
details and wood anatomy are difficult to deal with

Cow parsnip, 

Heracleum maximum

, has leaves with a pinnate 

vein pattern and is a dicot

Alumroot or 

Heuchera

leaves display a netlike veiin pattern

Irises, like 

I. confusus

are typical monocots with the main veins 

running parallel the length of the leaf

In some monocots, like calla lily (

Zantedeschia aethiopica

), the 

parallel veins come of a midrib, making the vein pattern 

potentially confusing

The leaves of salvia blanca, 

Sideritis

sp., are so densely covered 

with woolly hairs that the vein pattern is obscured. Sometimes 

rubbing the hairs off will reveal the veins.

The second part of a key is finding the family

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There are roughly 450 families of flowering plants in the 
world, with around 160 for California

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Because the majority of our natives belong to 40 to 50 diverse 
families, learning the field traits to recognize them helps 
greatly in the i.d. process

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For horticultural families, especially including tender indoor 
plants, the number of families is vaster and thus more difficult

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Families consist of anywhere from a single species to 20,000 
or more (orchids and composites)

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Most families have a cluster of 

key

features that aid in i.d.; 

we’ll be practicing those during the course

Once the family has been determined, it’s time to find the genus. 

Every family has one (

type

genus) to hundreds

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In California’s flora, there are many genera that you can also 
learn to recognize in the field including 

Eriogonum, 

Arctostaphylos, Ceanothus

, and 

Delphinium.

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In some families, the genera (plural of genus) are better 
defined but some overlap with each other (although very few 
genera are capable of crossing)

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Once the genus has been found out, the species or kinds need 
to be figured out. Some genera have a single species 
(monotypic) while others have hundreds of species, in which 
case keying to species can be difficult (especially when 
microscopic characters are used as in the Boraginaceae

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Species do often hybridize, making exact i.d. almost 
impossible in some groups like the oaks

Some genera can also be learned on sight, although sorting out 

the species may be difficult. Manzanitas (

Arctostaphylos

spp.) 

are identified by the shiny red bark and nodding, urn-shaped 

white or pink flowers

Another easy-to-recognize genus is 

Ceanothus

, which features 

hundreds of tiny flowers in dense spikelike clusters, the flowers 

with 5 colored sepals 

and

petals 

The genus 

Delphinium

(larkspur) has highly distinctive flowers 

that identify it. The sepals are showy and brightly colored with 

the upper sepals forming a pointed nectar spur

Although some species are very distinctive, many are variable, 

making it hard to know where to draw the line between one 

species and a close relative

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Species with consistent variations (flower size, habitat, flower color) are 
often subdivided into 

varieties

or 

subspecies

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Unfortunately, the distinction between these two categories are now 
blurred, so either one can apply to what I’m describing here

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These 

botanical 

varieties are not the same as varietal names used in most 

nurseries—their varieties are actually more accurately called 

cultivars

(short for cultivated variety), and most major groups are loaded with them

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Cultivars are genetic variations that appear in the wild, in a nursery, or 
someone’s garden that have at least one trait different from its sisters and 
brothers. Examples include flower color, double petals, resistance to frost, 
height of plant, and more

Next we’ll turn to classification, which refers to a system of 

placing plants for retrieval through i.d.

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Today, many factors determine the generation of classification 
systems, but first let’s take a look at what went on before

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Often the Greeks are credited with the start of classification 
systems, but we must remember our European bias; many 
other cultures had systems for grouping plants

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These first systems were what we now call “aritifical”, 
meaning they weren’t based on sound concepts of 
relationship and evolution

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Typical categories for these systems included (much as 
gardeners do today), tree, shrub, herbaceous, vine, etc.

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Such systems are practical especially because the known 
number of species was very small at that time

Many classification systems suffered inaccuracies when copied 

and little in the way of new ideas during the so-called Dark Ages

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As the Dark Ages ended, people looked anew at classification, 
now with an eye to similarity as an indication of relationship 
and thus position in the system

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One of the early popular ways of doing “natural” systems was 
counting number of flower parts, which is still done today

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As time went on, people more and more looked at all the 
obvious, morphological features to produce their version of a 
system

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Some weighted certain characters over others, again a 
practice that sometimes (often unconsciouly) is done today

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During this period, Carl Linnaeus invented a way of naming 
the plants in the systems, more about that later

As the concept of evolution came to light by Charles Darwin and 

others, there was a move to reflect which groups gave rise to 

other groups, an overview of evolution

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Many of the traits used in natural systems contributed but some new 
ideas were bandied about, such as which flowers are “primitive” and 
which “advanced”, an indication of when the groups evolved

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These systems are called 

phylogenetic

, and all systems since have been of 

this kind

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You might be tempted to think that we could figure out the evolution of 
plants but we still are working at it

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Most flowers do not fossilize, so depending on the fossil record, use in 
preparing these systems is seldom important

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Often only fragments of plants are fossilized, and we don’t really know if a 
leaf fragment goes with a wood fragment

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Thus, most of the basis for phylogenetic systems has been selecting 
characters that are supposedly ancestral and others that are derived. 
Specializations often indicate this idea.