Using a Dichotomous Key to identify plants

Using a Dichotomous Key to identify plantsThere are many different ways to identify a plant. Woody plants can be identified by bark, leaf shape, flower, or fruit. But to organize this massive amount of information for easy accessiblity presents a challenge. One way to organize these plant characteristics is by using a dichotomous key. A dichotomous key uses a series of contrasting statements to lead the reader to the correct identity for a given plant. These contrasting statements are always in sets of twos called couplets. Here is an example:

1a. Leaves are opposite or whorled…………………………21b. Leaves are alternate……………………………….……143

These two statements are a couplet because each statement refers to the same plant characteristic, in this case leaf arrangement. If the tree you are attempting to identify has opposite leaves, making 1a true, then the key directs you to couplet number 2. If the tree has alternate leaves, making 1b true, the key directs you to couplet 143. You will then go to that couplet and decide if either a or b is true for that couplet and then go to the next couplet. Usually it takes about 6-9 couplets to determine the identity of the plant you are looking at.

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Key to Common Woody Landscape Plants in the Midwest by Stidd and Henry is the dichotomous key we will be using in HORT 217. Because various botanical terms are used throughout the key, there is a glossy of terms starting on page 76. It will be helpful for you to have your text with you for this lesson.

Abbreviations used throughout the key are:Flr(s) ……Flowers(s)Frt(s)…….Fruits(s)lf………….Leaflvs………..Leaveslflet(s)……Leafletsyr(s)……...Year(s)

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This key uses the typical summer vegetative foliage to “key out” a plant. Flowers and fruits do not have to be present to use this key. But plants are only identified down to the species level. You will have to use Dirr’s Manual of Woody Landscape Plants to determine subspecies or cultivar.

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The key always starts on page 6 with the following:

IF PLANT IS A GYMNOSPERM (has seeds borne in cones – no flowers or fruits, has needle-like or scale-like leaves) GO TO NUMBER 1 ON PAGE 66

IF PLANT IS AN ANGIOSPERM (has flowers and fruits during the life cycle, has broad leaves with net venation) GO TO NUMBER 1 BELOW

We will now use our dichotomous key to identify the woody plant pictured here.

With your mouse left-click below on the correct answer for our woody plant pictured above:

GYMNOSPERM ANGIOSPERM

Gymnosperms have needle-like or scale-like leaves. This woody plant has broad leaves and veins in the leaf that fan out in a net-like manner. Gymnosperms are typically evergreen and do not lose their leaves in the fall. This plant is not gymnosperm.

With your mouse left-click on the red text to move forward

TRY AGAIN

Angiosperms have broad leaves and net-like veins just like this plant pictured here. This plant is an angiosperm. The key directs us to couplet 1 on page 6.

With your mouse left-click on the red text to move forward

GO TO COUPLET 1

Couplet 1

1a Leaves opposite or whorled (avoid young stems with unelongated internodes)…………………………………………………………………………2

1b Leaves alternate………………………………………………………………..143

With your mouse left-click on the red text for the correct answer

LEAVES ALTERNATE (1b)LEAVES OPPOSITE OR WHORLED (1a)

By comparing with the diagrams above, our woody plant does not have an opposite or whorled leaf arrangement. It is alternate.

With your mouse left-click on the red text to try again

TRY AGAIN

Opposite WhorledAlternate

You are correct. By comparing with the diagrams above you can see that our woody plant has an alternate leaf arrangement. Couplet 1b directs us to couplet 143 on page 25.

With your mouse left-click on the red text to go to the next couplet

GO TO COUPLET 143

Opposite WhorledAlternate

The next couplet requires us to know the difference between a leaf and a leaflet.

143a. Lvs simple……………………………………....181143b. Lvs compound…………………………………144

Left Click on the correct answer

Leaflet

Bud

Bud

Simple Leaf Compound LeafOur Woody Plant

Leaf is Simple – go to couplet 181 Leaf is Compound – go to couplet 144

Because there are buds at the base of each leaf, the leaves on our plant are simple, not compound. We now go to couplet 181 on page 29.

143a. Lvs simple……………………………………....181143b. Lvs compound…………………………………144

Leaflet

Bud

Buds

Simple Leaf Compound LeafOur Woody Plant

Leaves are Simple - Go to Couplet 181

Buds

You are correct. The leaves are simple. The next couplet is 181.

181a. Lvs scale-like, about 1/8” long……………………………....Tamarix181b. Lvs not scale-like, 1/4” or longer………………….………………182

The leaves on our woody plant are not scale-like and much longer than 1/8 inch. Our woody plant is not Tamarix. We now go to couplet 182.

Go to Couplet 182

Our Woody Plant

Scale-like leaves are foundon Gymnosperms

182a. Ground cover or vines………..……………………....184182b. Trees or shrubs…………………...……………………195

Our plant is a small tree or shrub. So we go to couplet 195 on page 31.

Go to Couplet 195

195a. Leaves entire………….………..……………………....196195b. Leaves toothed.…………………...……………………277

The leaves on our plant have an entire margin. Go to couplet 196

Go to Couplet 196

Our Woody Plant

196a. Stems armed…….…….………..……………………....197196b. Stems unarmed.…………………...……………………202

The stems on our woody plant are unarmed meaning there are no thorns or barbs.

Go to couplet 202

Go to Couplet 202

Our Woody Plant

202a. Leaves heart-shaped: flwrs rosey pink; frt a flattened pod…..……………………............Cercis canadensis202b. Leaves not heart-shaped: Frt not a pod.…….203

Our woody plant has heart shaped leaves, in the spring has rosey pink flowers and in the summer has pods. Our woody plant is Cercis canadensis. The common name is Eastern Redbud.

To Next Example

Heart-shaped Leaf Flowers rosey pink Fruits flattened pods

Again, we start on page 6 with the following:



We will now use our dichotomous key to identify a different woody plant.

With your mouse left-click on the correct answer:

GYMNOSPERM – Go to Couplet 1 on Page 66

ANGIOSPERM – Go to Couplet 1 below



Our Plant is a Gymnosperm – Go to Couplet 1 on Page 66

Our Woody Plant

Angiosperms have broad leaves and net-like venation. Angiosperms do not have needle-shaped leaves and cones. Our woody plant is a gymnosperm.

1a. Lvs broad, fan-shaped, parallel veins: Dioecious; cones absent, pollen produced in 1” long catkin-like structures; seeds stalked, borne singly, fleshy, green to yellow-orange, foul odor when crushed………………………………………Ginkgo biloba1b. Lvs and reproductive structures not as above: Cones present (sometimes highly modified)……………..2

1b is correct – Go to couplet 2

Our Woody Plantis a gymnosperm.

Ginkgo biloba is one of the few broad-leaf gymnosperms. The leaves have dichotomous venation, while angiosperms have net-like venation.

2a. Most lvs whorled or in clusters AND 1” or more long; Cones woody with overlapping scales……………………………………….32b. Lvs not BOTH clustered AND not 1” or more long: Cones woody or fleshy………………..21

For 2b to be correct, the needles would not be clustered and would be less than 1 inch long as pictured in the spruce.

The leaves of most gymnosperms are needle-shaped. Our woodyplant has needles 3-5 inches long in clusters of 5. The scales of the cone are woody and overlap each other.

2a is correct – go to couplet 3

Spruce

3a. Lvs in clusters of 5 or less: Cone scales bearing two seeds…………………….…….43b. Lvs in whorls or clusters or 10 or more: Scales with 2 or more seeds……………………………..17

Our woody plant has needles in clusters of 5. Each individual scale of the cone bears two seeds.

3a is correct – go to couplet 4

Scale

Seeds

4a. Lvs in clusters of 3 or mostly 3’s; bark exfoliating creating irregular brownish to whitish patches: Cones sessile, 2-3” long, 2” across, yellowish brown, recurved spine at tip of scale……………………………….Pinus bungeana4b. Lvs in clusters of 2 or 5……..………………………………5

Our woody plant has needles in clusters of 5. Bark does not exfoliate. Cones are not sessile, but instead are attached to the tree with a stalk

4b is correct – go to couplet 5

5a. Lvs in 2’s………………………………………………………65b. Lvs in 5’s……………………………..……………………...12

Our woody plant has needles in clusters of 5.


12a. Lvs 5-8” long: Cones with 1-2” stalk, up to 10” long by 2” wide, resinous……..…………………………………Pinus wallichiana12b. Lvs 5” or less……………………………..……………………...13

The leaves of our woody plant (needles) are 3-5 inches long. The stalk attaching the cone is only ½ inch long.


13a. Lvs 1 ¾ “ or less: Cones nearly sessile, to 4” long prickles ¼” and curved………..……Pinus aristata13b. Lvs longer…..……………………………..……………………...14


The leaves of our woody plant are much longer than 1 ¾ inches. Because there is a stalk which attaches the cone to the tree, the cones are not sessile.

14a. Lf edges smooth to touch; young stems shiny green and extremely flexible (can be tied in knots): Cones short stalked, to 6” long (10”), resinous………..………..………Pinus flexilis14b. Lf edges minutely toothed……………..……………………...15

The needles of our tree have minutely toothed edges which can be felt as a roughness much like a fine-grained sandpaper when the needles are grasped.


15a. Lvs soft and flexible; stems with tufts of short hairs subtending leaf clusters: Cones stalked, to 8” long often curved, pendent, prickles present………………………..Pinus strobus15b. Lvs rather stiff; stem pubescence not as above………..….16

15a is correct – Our woody plant is Pinus strobus. The common name is Eastern White Pine.

Needles of our tree are soft and flexible. Tufts of short hairs appear at the base of needle clusters. The curved, prickly cones are pendent (meaning hanging downward).

Tufts of short hairs

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Using a Dichotomous Key to identify plants