25.2: Dichotomous Key to Common Fruits - Biology

25.2: Dichotomous Key to Common Fruits - Biology

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If the fleshy part of the fruit is composed of something other than the ovary, it is an accessory fruit.

1. Fruit from one carpel (can be several fused together) of one flower ........[Simple Fruit].... 2

1. Fruit from more than one free carpel in a single flower or from an inflorescence .... 16

2. Fleshy at maturity ....................................... 3

2. Dry at maturity ............................................ 8

Fleshy Fruits

3. Thin exocarp, fleshy mesocarp, stony endocarp surrounding single, large seed ............... Drupe

3. Fruit not as described above............................................................................................... 4

4. Seeds in a linear order, separate from ovary wall, pericarp splits on two seams ...Legume (immature)

4. Fruit not as described above.................................................................................. 5

5. Papery endocarp forms a core. Derived from a perigynous flower..........Pome

5. Endocarp fleshy (not a papery core) ...................................................... 6

6. Thin exocarp, fleshy mesocarp, one to many seeds ........................... Berry

6. Exocarp thickened and leathery (modified berries) ………......... 7

7. Exocarp and mesocarp form leathery rind, locules filled with juice-filled trichomes ……...Hesperidium

7. Exocarp forms tough skin/rind, thick mesocarp, not divided into separate locules ........... Pepo

Dry Fruits

8. Dehiscent (splits open at maturity), usually many seeds ....... 9

8. Indehiscent (does not split open), usually one seeded .......... 12

9. Derived from a carpel with one locule ............................. 10

9. Derived from a carpel with more than one locule ............ 11

10. Dehiscent along one seam .......................... Follicle

10. Dehiscent along two seams ........................ Legume

11. From two locules with a central partition .............. Silique (elongate) or silicle (round)

11. From more than two locules.................................. Capsule

12. Ovary wall extends to form a wing .................. Samara

12. Fruit not winged ............................................... 13

13. Outer wall not especially thick or hard, seed small ... 14

13. Outer wall hardened, seed relatively large ................ 15

14. Seed not tightly attached to pericarp ............... Achene

14. Seed fused to pericarp, grains ........................ Caryopsis

15. Stony pericarp surrounds one large seed .............................................. Nut

15. Relatively thin exocarp, fibrous mesocarp, single large seed .............. (dry) Drupe

Aggregates & Multiple Fruits

16. Derived from one flower with many free carpels ...... Aggregate Fruit

16. Derived from an inflorescence (many florets) ........... Multiple Fruit

Classification of Fruit Types

The structure known as a "fruit" is found only in the members of the Angiosperms. A fruit developed solely from the ovary and its contents is known as a true fruit. A fruit developed from the ovary and its contents plus additional parts of the flower such as the receptacle, petals, and sepals is known as an accessory fruit (e.g. pineapple). The following is a common classification of fruit types. (see pp. 489-493 in Plant Systematics, 2nd ed., for additional information).

I. Simple Fruits - Fruits formed from 1 pistil. They may be either true or accessory fruits.

A. Dry Fruits : - Fruits in which the coat becomes dry at maturity.

1. Dehiscent Fruits - Dry fruits which at maturity open by definite natural means to shed the contained seeds.

i. Legume A dry dehiscent fruit developed from 1 carpel and at maturity splitting along both the dorsal and ventral sutures. (beans, peas).

ii. Follicle A dry dehiscent fruit developed from 1 carpel and at maturity splitting along only one suture. (larkspur, columbine)

iii. Capsule A dry dehiscent fruit developed from several carpels.

(a) Loculicidal capsule - one which splits along the outer median line. (lilies).

(b) Septicidal capsule - one which splits along the septa and opens at the top. (yucca, agave).

(c) Silique - a special long slender capsule of 2 carpels. (mustards).

(d) Silicle - a special short broad capsule of 2 carpels. (mustards).

(e) Pyxis - a capsule which has circumscissle dehiscence. (plantain, amaranths, purslane).

(f) Poricidal capsule - one which opens with round holes. (poppies).

2. Indehiscent fruits - Dry fruits which do not open when mature to shed their seeds. Many of this group are one seeded fruits.

i. Achene - A one-seeded, dry, indehiscent fruit the one seed is attached to the fruit wall at a single point. (buttercups, dandelion, sunflower).

ii. Nut - A dry, indehiscent, one seeded fruit similar to an achene but with the wall greatly thickened and hardened. (beech, chestnut, oak, hazel walnut and hickory - note: because of extrafloral bracts, or "husk", the latter two fruits are sometimes called "drupes").

iii. Samara - A one- or two-seeded dry, indehiscent fruit in which part of the fruit wall grows out into a wing. (elm, maple, ash).

iv. Grain - A one-seeded dry, indehiscent fruit in which the fruit wall and the seed coat are fused. (wheat, corn, grasses).

v. Schizocarp - A fruit formed from several carpels, each carpel of this pistil enclosing a single ovule, at maturity the carpels separate as separate indehiscent fruits. (mallow, wild carrot, dill).

B. Fleshy Fruits - A fruit in which the wall becomes soft and fleshy as it matures.

1. Drupe - A one-seeded simple fruit developed from a superior ovary in which the innermost portion of the wall (endocarp) becomes hard and stony, the outermost part (exocarp) becomes a relatively thin skin, and the middle portion between the skin and the stone (mesocarp) becomes either fleshy or fibrous. (cherry, coconut, walnut and hickory - note: because of extrafloral bracts, or "husk", the latter two fruits are sometimes called "drupes", but best called "nuts").

2. Berry - A simple fruit in which the ovary wall or at least its inner portions become enlarged and usually juicy. (grape, banana, gooseberry).

Two special types of berry-like fruits may be singled out for special consideration.

(a) Hesperidium - This is a special type of berry in which a leathery rind forms the interior of the fruit divided by septa, indicating the number of carpels. (citrus).

(b) Pepo - This is a special type of berry in which a relatively hard rind is formed the interior of fruit not divided by septa. (watermelon, gourds, squash).

3. Pome - An accessory fleshy fruit formed by a group of carpels more or less firmly united with each other and surrounded by and united to the floral tube or receptacle. (apple, pear, mountain ash).

II. Aggregate Fruit - A fruit formed by the development of a number of pistils from the same flower. The individual units may be berries or other specific types. (raspberry, strawberry).

III. Multiple Fruit - A fruit formed by the development of a number of pistils often with accessory parts, the pistils being from a number of flowers. (mulberry, fig).


This investigation requires students to create and test a model for fruit classification. It may be used as an introductory inquiry into taxonomy for life science classes or as an NGSS Dimension 1 Practice (developing and using models) in life science, botany, or agricultural science classes. Students observe the exterior of common fruits, group fruits based on observations, dissect the fruits, and then regroup as necessary. Students will convert the classification model into a true dichotomous key. Students will evaluate the dichotomous keys by classifying 2 additional fruits.

Curriculum Resources for Michigan Agriculture Teachers


Demonstrate the use of a dichotomous key to identify plant species by traits and specific characteristics such as leaf, bark, bud and others. Show major categorical differences between gymnosperms and angiosperms. Review the seven taxonomic levels of classification, which are kingdom, phylum, class, order, family, genus and species.


    (Biology Dictionary)
  • Common trees of Michigan or Fruit Tree online keys
  • Self design own dichotomous key using devices or things around the home
  • Youtube videos on various dichotomous keys (American Museum of Natural History) (ZME Science)
  • Dichotomous Key to ID Invasive Species (Alice Ferguson Foundation)

Teaching Methods

Brainstorming, Lecture Discussion, Paired or Small Group Discussion, Case Study, Field Trip, Resource People, Game and Simulations, Supervised Study

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An Overview on Plant Identification

The below mentioned article provides an overview on Plant Identification:- 1. Meaning of Plant Identification 2. Methods of Plant Identification 3. Plant Characters before Its Identification 4. Keys 5. Recent Methods 6. Artificial Keys.

Meaning of Plant Identification:

Plant identification implies assigning a plant to a particular taxonomic group – ultimately to the species. The identification of plant specimen is its determination of as being identical with or similar to another and already known plant.

Identification is the determination of a taxon as being identical with or similar to another and already known elements the determination may or may not be arrived at by the aid of literature or by comparison with plant of known identity. No names need be involved in the process of identifying a plant. The naming of plant or nomenclature is different than identification.

When the unknown plant is collected from a known locality, the common practice is to refer to a book accounting for the plants of that region. This contains usually the analytical keys and descriptions.

Methods of Plant Identification:

The first step is the determination of the families to which the unknown plant belongs. Knowing the name of the family one can turn the keys to genera for determining the generic name and then for the specific identity of the plant to the species key.

Since, for many reasons, the identity and name of the plant obtained may be incorrect, it is always safe to check the description of the plant to ensure that there is a reasonable agreement between the characters observed in the unknown plant and those given in the description of the plant presumed to be.

Second method is the utilization of the latest floras and check list of the particular region. These comprise usually an index to the plants known for the locality and generally provide other pertinent habit, distributional and frequency data.

By the process of elimination an unknown plant can be assigned to a genera having one or more species, and identification may be completed by comparison of characters with those given in any standard work accounting of the plants of that area.

Third method is the identification by means of monographs or revisionary works accounting for the particular family or genus.

Plant Characters before Its Identification:

Study the plant specimen to be identified in detail.

Mention the following characters:

1. Nature of specimen – herbaceous, or woody annual or perennial.

2. Phyllotaxy and venation.

3. Inflorescence type – Capitulum (e.g. Asteraceae), Cyathium (e.g. Euphorbiaceae), Verticellaster (e.g. Lamiaceae) etc.

4. Flower and its parts – actinomorphic or zygomorphic.

5. Presence of epicalyx (e.g. Malvaceae).

6. Number of sepals and petals or tepals, their aestivation.

7. Petals free (e.g. polypetalae) or fused (e.g. gamopetalae).

8. Number of stamens and their position – antipetalous (e.g. Chenopodiaecae) alternipetalous or obdiplostemonous, (e.g. Caryophyllaceae). Staminal tube (e.g. Malvaceae).

9. Count number of carpel/carpels, style – gynobasic (e.g. Lamiaceae) stigmas.

Keys in Plant Identification:

A key is a device for easily identifying an unknown plant by a sequence of choices between two or more statements. A key is an artificial analytical device or arrangement where by a choice is provided between two contradictory characters resulting in the acceptance of one and the rejection of the other. Statements in the keys are based on the characters of the plants (mentioned above).

For example, a key might separate taxa using the following choices:

(1) Herbaceous versus woody if herbaceous, the woody plants are eliminated

(2) The next choice, zygomorphic flowers versus actinomorphic, if zygomorphic, the plants with actinomorphic flowers are eliminated and so forth.

Each time a choice is made, the number of taxa that remain is reduced by the use of contrasting characters. If sufficient numbers of characters are contrasted, the number of possibilities is eventually reduced to one.

Keys used in floras are usually diagnostic, that is, identifying an unknown plant by the conspicuous features. Diagnostic characters are sometimes referred to as key characters. A key may be short and limited to a single pair of contradictory statements or propositions. A single pair of contradictory statements or each pair of choices is called a ‘couplet’.

Each statement of a couplet in a key is called ‘Lead’. Keys do not offer descriptions of the plants concerned, but state only the essential diagnostic characters by means of which the taxa can be identified.

Keys are of two types: punched card keys and dichotomous keys.

Punched Card Keys:

Punched card keys are used in the school, colleges etc. by the students. Punched card keys consist of cards of suitable size with names of all the taxa (all families, genera or species for which the key is meant) printed on each one of them.

Each card has a number and any one character printed near one of the corners. All the taxa showing this character are indicated by a perforation in front of their names, while those lacking this character are without any perforation.

Dichotomous Keys:

A dichotomous key presents two contrasting choices or couplet at each step. The key is designed so that one part of the couplet will be accepted and the other rejected. The first contrasting characters in each couplet are referred to as the primary key characters. These are usually the best contrasting characters. Characters following the lead are secondary key characters.

The dichotomous keys are of two types, viz., Indented key (Yoked key) and Bracketed key (Parallel key).

Indented key or Yoked key:

A dichotomous key in which the first part of a contrasting couplet is followed by all subsequent couplets each subordinate couplet being indented one step further to the right for clarity of presentation. The indented key is the one most widely used in manuals for the identification of vascular plants. In the indented key, each of the couplets is indented a fixed distance from the left margin of the page.

Bracketed or Bracket Key or Parallel Key:

A dichotomous key in which contrasting parts of a couplet are numbered and presented together, without intervening couplets, although the brackets joining each couplet are now omitted.

The plants used in the example are common genera of the family: Ranunculaceae, viz., Clematis, Anemone, Ranunculus, Aquilegia and Delphinium.

The first choice, with in the above genera is between “Fruit a group of achenes flowers not spurred” and “Fruit a group of follicles flowers spurred”, these paired statements being given the same indention.

If the latter choice is taken, the next choice, shown of the indention, is between “Flowers regular, spurs 5” and “Flowers irregular, spur 𔃱’. Thus the plant in question has follicles and irregular flowers with a single spur, it must be a Delphinium.

B. Bracketed Key or Parallel Key:

In this the two couplets are always next to each other in consecutive lines on the page.

The same example of bracketed key is given below:

(i) Fruit a group of achenes flowers unspurred (2)

(i) Fruit a group of follicles flowers spurred ……… (4)

(3) Sepals usually 4 involucre absent………. Clematis

(3) Sepals usually 5 involucre present……… Anemone

(4) Flowers irregular spur 1 …………. Delphinium

The number at the right end of a line in the bracket key indicates the next numbered pair of choices to be considered.

The keys use the most conspicuous and clear-cut characters, without special regard to those considered taxonomically the most important. For this reason the sequence of taxa is often quite artificial, and such keys are frequently termed artificial keys. Artificial key is an identification key based on convenient phenotypic characters and not indicating phylogenetic relationships.

Natural key is an identification key constructed from a natural classification and indicating the supposed evolutionary relationships of the group within the branching sequences of the key.

Comparison of Indented Key and Bracketed Key:

1. Each couplet has its 2 leads indented by the same amount from the left-hand margin of the page.

2. The first couplet to be consulted is the one least indented and which has its first lead at the head of the key.

3. The next appropriate couplet to be consulted is the one with its first lead immediately below the chosen lead of the previous couplet, its leads being the next least indented pair below the latter.

1. Each couplet has its 2 leads immediately adjacent under the same left-hand number.

2. The first couplet to be consulted stands at the head of the key next to the number 1.

3. The next appropriate couplet to be consulted is indicated by the reference number to further down the key, placed on the right-hand side of the chosen lead.

Construction of Key:

In constructing a key following techniques may be followed:

1. Key should be dichotomous.

2. The first word of each lead of the couplet should be identical. For example, if the first lead of a couplet begins with the word fruit, the second lead of the same couplet must begin with the word fruit as in example.

3. The two parts of the couplet should be made up of contradictory statements so that one part will apply and the other part will not i.e., rejected.

4. Do not use overlapping ranges or vague generalities in the couplets.

5. The couplets should be of positive statement e.g., “leaves narrow versus leaves not narrow”.

6. Use distinct and readily observable features.

7. The leads of consecutive couplets should not begin with the same word, since this may cause confusion.

8. It may be necessary to provide two sets of keys in some groups flowering versus fruiting material, vegetative versus flowering, or staminate versus pistillate for dioecious plants.

9. Couplets of a key may be numbered or lettered, or may use some combination of lettering and numbering, or may be left blank in the case of indented keys.

Keys are traditional method of identification in taxonomy. If keys are well written with adequate specimens and carefully, then the specimen can be successfully identified. Keys, however, have several major disadvantages. The use of certain characters is required even if the character is not evident in the unknown specimen.

Recent Methods of Identification:

Attempts to improve on the traditional identification process by using keys have resorted to either polyclaves or computer techniques for plant identification. These methods provide multientry (or multiaccess) opportunities that is, the place of entry is not fixed by the format of the key.

Polyclave Identification:

A polyclave is a multientry, order-free key implemented in one of several different formats. One form is a diagnostic key that uses cards placed on top of one another to eliminate taxa that disagree with the specimen to be identified.

Polyclaves have a tremendous advantage over dichotomous system of plant identification. In this system the user is free to select appropriate characters for each unknown specimen. The route taken to a particular taxon differ considerably from one plant specimen to another.

The logic of identification with a polyclave is the same as that in a key, but the user is free to choose any character, in any sequence, thus avoiding the rigid format of traditional keys.

The polyclaves in existence are (a) cards with holes commonly referred to as “peck-a-boo” or “window” cards (b) edge-punched or “key sort” cards and (c) semi-transparent overlays.

(a) Peck-a-boo or Window Card Key:

The peck-a-boo or window card key to World angiosperm families uses a card for each character. Round holes are punched beside a family number for the families that have that character. The plant to be identified is examined and its characters noted. With a good specimen, it is possible to find 20 to 25 characters.

One selects those cards corresponding to the characters found on the plant. The cards are then put on top of one another in a stack. Families having all the observed characters will be indicated by the holes, or “windows”, which are easily seen when the cards are held in front of a light. The logic is simple. Each time a card is added, families not perforated in this new card are excluded.

Computerised Identification:

The plant identification by computer programme system has been introduced by Morse (1974).

The computerised efforts of plant identification are grouped into:

(a) computer-constructed keys

(b) computer stored dichotomous keys

(c) automated pattern recognition systems and

(d) simultaneous character-set methods.

Recently polyclaves may be pouched by computer or standard computer cards.

Two general kinds of computer-based polyclaves have been developed. One kind developed for qualitative taxon-character data, employs elimination.

The other, developed for use of taxon-character frequency tables, employees likelihood ratios or other probabilistic techniques. Another form is a computer-stored multi-entry key. Still another polyclave is a printed table or matrix giving the status of various taxa and characters useful for separating the taxa.

Artificial Keys for The Identification of Major Groups:

Dome-shaped unexpanded thalamus polysepalous hypogynous ovary superior.

Thalamus expanded into a disc ovary superior.

Calyx gamosepalous thalamus cup-shaped disc thin, often lining the calyx tube.

Petals fused ovary inferior.

Petals fused flowers with calyx and corolla ovary superior two carpels.