Plant Science Notes

Dichotomous Key and Species Identification

Introduction to Dichotomous Keys

  • A dichotomous key is a critical tool used to identify items and organisms in the natural world, offering a systematic approach to classification.

  • It's a widely used classification method in biological sciences, facilitating quick and easy identification of unknown organisms based on observable characteristics. Its simplicity and efficiency make it invaluable for both novice and expert users.

  • Keys consist of a series of choices that lead to the correct identification of an item. Each choice is a carefully constructed statement about the organism's attributes.

  • "Dichotomous" means "divided into two parts," so these keys always present two choices at each step, ensuring a binary decision-making process.

  • The user is presented with two statements based on the organism's characteristics at each step. These characteristics can range from physical appearance to specific behaviors or chemical properties.

  • Correct choices at each step will lead to the organism's name, providing a clear path to identification.

  • Avoid hasty decisions based on a single feature; verify multiple characteristics for accuracy. Confirmation through multiple traits ensures reliability and reduces errors.

Recommended Field Guides

  • Field Guide to Trees of Southern Africa by Van Wyk, B & Van Wyk, P. (2013), pages 15-17. This resource provides detailed descriptions and illustrations for tree identification.

  • Field Guide to Fynbos by Manning, J. (2018), pages 26-31. Offers insights into the diverse plant species found in the Fynbos region.

Physiological Characteristics for Tree Identification

  • Use pages 705-709 in Field Guide to Trees of Southern Africa (Van Wyk, B. and van Wyk, P. 2013) along with Unit 3. This section covers essential physiological traits that aid in tree classification.

Characteristics Used in Tree Classification

  • Botanists primarily use floral structure and fruit for identification because these reproductive parts provide definitive characteristics. However, relying solely on these can be limiting.

  • However, flowers and fruit are not always present, so other characteristics are also essential. This necessitates the use of vegetative features like leaves, bark, and overall tree shape for year-round identification.

Key Identifying Characteristics

  • Flowers (van Wyk & van Wyk, 2011). Floral characteristics include color, shape, size, and arrangement, which are vital for species differentiation.

  • Fruits and seeds (van Wyk & van Wyk, 2011). Fruit morphology, seed structure, and dispersal mechanisms provide additional clues for identification.

  • Overall impression and shape of the tree ("Jizz" - tall, spreading, drooping, shrubby, etc.) (van Wyk & van Wyk, 2011). The general growth habit and form of the tree can often provide an immediate clue to its identity.

  • Leaves (simple, compound, smooth, hairy, discolourous, etc.) (van Wyk & van Wyk, 2011). Leaf characteristics such as shape, size, margin, surface texture, and venation patterns are crucial for distinguishing species.

  • Bark (dark, light, smooth, blocky, etc.) (van Wyk & van Wyk, 2011). Bark texture, color, and pattern can vary significantly between species and with age.

  • Thorns and spines (white, brown, straight, curved, etc.) (van Wyk & van Wyk, 2011). The presence, type, and arrangement of thorns and spines help differentiate species, especially in arid environments.

  • Latex (milky, watery, etc.) (van Wyk & van Wyk, 2011). The presence and nature of latex can be diagnostic for certain plant families and genera.

  • Other characteristics (single-stemmed, multi-stemmed, etc.) (van Wyk & van Wyk, 2011). Growth form, branching patterns, and other unique features contribute to accurate identification.

Canopy

  • The canopy is the upper part of the tree, formed by the branchlets/twigs and leaves (Grant & Thomas, 2000). It is the tree's primary interface with sunlight and the atmosphere.

Different Canopy Types Include:

  • Wide spreading (e.g., Combretum erythrophyllum). This type maximizes sunlight capture, often shading the ground below.

  • V-shaped (e.g., Boscia albitrunca). A distinctive form that can aid in quick visual identification.

  • Narrow (e.g., Young tamboti). Common in trees adapted to dense forests or competitive environments.

  • Irregular (e.g., Croton sylvaticus). This canopy type is often the result of environmental factors or growth patterns.

Trunk and Stem

  • The trunk is used for larger trees, while stems are for smaller and/or multi-stemmed trees (Grant & Thomas, 2000). The trunk provides structural support and vascular transport, while stems perform similar functions in smaller plants.

Different Trunk and Stem Types Include:

  • Multi-stemmed (e.g., Dichrostachys cinerea). This growth form is often an adaptation to fire or herbivory.

  • Single-trunked, low-branching (e.g., Ficus ingens). This form provides stability and access to sunlight.

  • Straight, single-trunked, high-branching (e.g., Burkea africana). Common in forests where competition for light is intense.

  • Crooked trunk (e.g., Combretum molle). Often the result of environmental stress or physical damage.

  • Fluted trunk (e.g., Olea europaea). These trunks have ridges and grooves, providing added structural support.

  • Buttressed trunk (e.g., Ficus sur). Buttresses provide additional stability, especially in shallow or waterlogged soils.

Bark

  • Bark texture and color are often characteristic, but they can differ between younger and older trees (Grant & Thomas, 2000). Variations in bark can be due to age, environmental factors, and genetic differences.

Leaves

  • Softwoods usually have needles, while hardwoods usually have leaves (van Wyk & van Wyk, 2011). This is a general rule, though there are exceptions.

Simple Leaves

  • A simple leaf has a single leaf blade attached to a petiole, which attaches to a twig (van Wyk & van Wyk, 2011). The leaf blade is the main photosynthetic surface of the leaf.

  • There is a leaf bud at the base of the leaf stalk. This bud can develop into a new leaf or branch.

  • Lobed leaves are deeply divided but not completely separated. Lobes increase the leaf's surface area for sunlight capture.

Compound Leaves

  • A compound leaf is made up of two or more leaflets or pinnae (van Wyk & van Wyk, 2013). Compound leaves can reduce wind resistance and herbivore damage.

  • Leaflets do not have a leaf bud, only leaf stalks do. This distinguishes leaflets from individual leaves.

Pinnately Compound Leaves

  • Leaflets are arranged oppositely or alternately along a common axis, the rachis (van Wyk & van Wyk, 2013). This arrangement maximizes light exposure for each leaflet.

  • Paripinnate: A once-pinnately compound leaf with leaflets paired along the rachis and no leaflet at the tip. This provides a symmetrical appearance.

  • Imparipinnate: Has a single leaflet at the rachis tip (odd number of leaflets) (van Wyk & van Wyk, 2013). This arrangement is asymmetrical.

Bipinnately Compound Leaves

  • Each leaflet (pinna) is divided into several parts (van Wyk & van Wyk, 2013). This creates a highly divided leaf structure.

  • Also called “twice compound”. This term describes the leaf structure in detail.

  • Examples: South African Senegalia and Vachellia spp.

Leaflet Shapes

  • Bifoliate: A compound leaf with two leaflets (van Wyk & van Wyk, 2013). Example: Colophospermum mopane.

  • Trifoliate: A compound leaf with three leaflets (van Wyk & van Wyk, 2013). Example: Searsia (Rhus) spp.

  • Palmately Compound: Several leaflets attached to the same point on the petiole (van Wyk & van Wyk, 2013). Example: Vitex rehmannii.

Leaf Arrangement

  • Alternate: One leaf per node, at different heights on the stem. Examples: Strychnos spp., Grewia spp., Ficus spp.

  • Opposite: Two leaves (a pair) per node, growing at the same level on opposite sides of the stem. Example: Ziziphus mucronata.

  • Whorled: Three or more leaves per node, forming an encircling ring. Example: Ozoroa paniculosa.

  • Tufted: Growing in a bundle, tuft, or close cluster. Example: Terminalia sericea.

Leaf Margins

  • The edge of the leaf varies from smooth to toothed to undulating (Van Der Schijff, 2007). Margin characteristics are important for species identification.

Leaf Venation

  • Some trees have leaves with prominent and characteristic venation.

Types of Leaf Venation

  • Single midvein. Examples: Mimusops zeyheri, Ochna spp.

  • Three or more veins from the base. Examples: Strychnos spp., Dombeya spp., Ozoroa spp.

  • Parallel venation. Examples: Ficus spp., Grewia spp.

Pods

  • Pods are dry and many-seeded dehiscent fruits (Grant & Thomas, 2000). Pods protect seeds and aid in their dispersal.

  • They are envelopes covering a seed or several seeds.

Latex

  • Latex is a liquid exudate, either milky or clear (van Wyk & van Wyk, 2013). Latex serves various functions, including defense against herbivores.

  • Milky latex: Moraceae (Fig) and Euphorbiaceae (Euphorbia) families. This type of latex is often toxic.

  • Watery latex: Sclerocarya birrea (Marula) and Lannea discolour (Live long). Watery latex is less common but still diagnostic.

  • Refer to page 707 in Field Guide to Trees of Southern Africa (Van Wyk, B & Van Wyk, P. 2013) for images.

Thorns, Spines, and Prickles

  • Spines: Modified branches from dwarf lateral shoots; leaves can grow from them. Examples: Dichrostachys cinerea, Gymnosporia buxifolia, and Carissa bispinosa. Spines are lignified and sharp, providing defense.

  • Thorns: Modified stipules. Examples: Vachellia spp. / Senegalia spp. Thorns are also sharp and provide protection.

  • Prickles: Small, sharp outgrowths of the epidermis or bark. Example: Zanthoxylum capense. Prickles are less robust than spines and thorns.

How to Identify Grass

  • Reference: Guide to grasses of southern Africa by Van Oudtshoorn, F. (2012).

Characteristics of Grasses

  • Round, hollow, or pithy jointed stems (culms) (Fish et al., 2015; Van Oudtshoorn, 2012). Culms provide structural support and nutrient transport.

  • Narrow sheathing leaves with parallel veins (Fish et al., 2015; Van Oudtshoorn, 2012). Parallel venation is a key characteristic of grasses.

  • Leaves alternate on two sides of the stem (Fish et al., 2015; Van Oudtshoorn, 2012). This arrangement optimizes light capture.

  • The junction of the blade and sheath often has an erect fringe of hairs (ligules) and sometimes earlike projections (auricles) (Fish et al., 2015; Van Oudtshoorn, 2012). Ligules and auricles are important diagnostic features.

  • Flowers are borne in reduced spikes (spikelets) (Fish et al., 2015; Van Oudtshoorn, 2012). Spikelets are the basic flowering units in grasses.

Distinguishing Grasses

  • Grasses must be distinguished from sedges (Cyperaceae), rushes (Juncaceae), restios (Restionaceae), and forbs.

Comparison

  • Sedge: Triangular stem, often grows in wet areas (Van Oudtshoorn, 2012).

  • Rush: Solid stem, parallel-veined leaves (Van Oudtshoorn, 2012).

  • Forb: Non-woody, non-grass plants with solid stems and broad, net-veined leaves (Van Oudtshoorn, 2012).

  • Grass: Rounded, hollow, jointed stems, parallel-veined leaves (Van Oudtshoorn, 2012).

Inflorescence

  • The whole flowering portion of a plant (Van Oudtshoorn, 2012).

  • Inflorescences vary in shape and size.

  • Best identified during the flowering stage.

Inflorescence Types

  • Unbranched: Solitary, Spike-like (e.g., Sporobolus africanus).

  • Compact: Unilateral.

  • Digitate: (e.g., Urochloa mosambicensis), Paired, Semidigitate, Spaced.

  • Paniculate: (e.g., Eragrostis lehmanniana), Contacted, Open, Drooping, Grouped.

Spikelet

  • The flower-bearing parts of the plant (Van Oudtshoorn, 2012).

  • The general structure is consistent within a genus.

Spikelet Structures

  • Single-flowered spikelet

  • Multi-flowered spikelet

Florets

  • Illustrations show one floret, two florets, and numerous florets (Fish et al., 2015).

Culm

  • Central axis of the mature grass shoot, comprised of nodes and internodes; each node bearing a leaf (Van Oudtshoorn, 2012).

Culm Features

  • Length of the culm (Van Oudtshoorn, 2012).

  • Growth form: e.g., geniculate - bent at the bottom nodes (e.g., Eragrostis tricophora).

Leaf Blade

  • A flat, thin outgrowth of a stem that is usually green; the principal area for photosynthesis (Van Oudtshoorn, 2012).

Leaf Blade Characteristics

  • Shape: Linear, Lanceolate, Ovate.

  • Form: Open, Folded, Rolled.

  • Hairiness.

Leaf Sheath

  • The lower part of the grass leaf which wraps around the stem.

Ligule

  • A thin, membranous outgrowth or row of hairs on the upper and inner side of the leaf blade where it joins the sheath.

  • May protect the leaf sheath from water or insect entry.

Ligule Characteristics

  • Form: Membrane, Membrane fringed with hair/hairy margin (Heteropogon contortus), Membrane a ring of hairs (Panicum maximum, Urochloa mosambicensis).

Node

  • May be surrounded by hairs (e.g., Dichanthium annulatum or Bothriochloa insculpta).

  • May be sticky (e.g., Eragrostis gummiflua).

Internode

  • The part of the stem between two successive nodes.

Root

  • The underground part of a plant that absorbs water, stores nutrients, and provides anchoring support in the soil.

Stolon

  • A stem that grows horizontally along the surface of the ground

  • these ‘runners’ take root at joints or tips, forming new plants


  • Rhizome
    A thickened underground stem that can produce new shoots and roots, serving as a means of vegetative reproduction. grows horizontally and can also store nutrients, allowing the plant to thrive in various environmental conditions.


sessile

  • Referring to plants or organs that are attached directly by the base without a stalk, often found in species that prefer stable positions for nutrient absorption.