Botanists seem to have invented a special term for every conceivable structure or arrangement of parts, no matter how small the size or slight the difference! The number of technical terms used in RainforestPlants has been kept mercifully small. Really! As you continue with your study of plants, your botanical vocabulary will undoubtedly continue to expand. Until then, mastery of the following terms and those contained in “How to Examine Plant” is a reasonable goal for the beginning tropical biologist.
Flowering plants have traditionally been divided into two large classes, the monocots and dicots. The names are derived from the numbers of “seed leaves” or cotyledons exhibited by newly emerged seedlings with monocots having one and dicots two. Monocots tend to have strap-like leaves with parallel venation. In the tropics they may also have large banana-like leaves and/or pinnate venation. Grasses, Palms, Arums, and Orchids are large monocot families. Dicots tend to have broad leaves with complex netted venation. All trees, except for the palms, belong in this group. The most recent classifications maintain these two classes, but recognize a third group, “Magnoliids,” formerly considered to be dicots. The separation is based upon closer analysis of morphological traits that are intermediate between the two older classes, despite having large leaves that clearly resemble dicots. The decision is also supported by DNA analysis. The “Magnoliids” are well represented in the tropics by the Annonaceae, Myristicaceae, Lauraceae, and Piperaceae families.
Blade or lamina – flattened part of leaf used to harvest light energy for photosynthesis
Margin – refers to the edge of a leaf blade. Usually entire (smooth), serrate (toothed), or crenate (scalloped).
Petiole – leaf stalk
Petiolule – leaflet stalk of a compound leaf
Rachis – part of a pinnately compound leaf formed by a continuation of the petiole. The leaflets attach to the rachis.
Simple leaves – consist of a single blade or lamina and are usually attached to the stem by a petiole.
Compound leaves – consist of multiple blades or leaflets and are palmate or pinnate. Compound leaves are much more common in the tropics than in temperate zones.
Palmate - hand-like compound leaf with leaflets radiating from a single point
Pinnate – feathery compound leaves with multiple leaflets coming off of an elongated continuation of the petiole known as a rachis. May be once or twice pinnate.
Monopinnate – rachis with leaflets coming off of it. May be paripinnate or imparipinnate.
Paripinnate - an even number of leaflets occurring in pairs along the rachis of a pinnately compound leaf.
Imparipinnate - an odd number of leaflets with a terminal leaflet at the end of the rachis of a pinnately compound leaf. Common in Burseraceae and Fabaceae-Papillionoideae.
Bipinnate – compound leaf in which the leaflets themselves are pinnate. Common in Mimosoid legumes (Fabaceae-Mimosoideae).
Veins – show up as ribs or lines radiating throughout the leaf. They are really clusters of vascular tissue (xylem which brings water and nutrients up from the ground and phloem which distributes the carbohydrates produced by photosynthesis throughout the plant) and associated structural fibers. Most broad leaves (dicots) have a prominent mid-vein with smaller veins branching off of it. Veins are connected to other smaller veins, such that no cell in the leaf is less than a few cells away from a vascular connection. Veins are referred to as primary, secondary, and tertiary and represent a continuum from larger to successively smaller diameters.
Venation – the arrangement of veins within a leaf. Note the non-intuitive spelling.
Parallel venation – veins are more or less parallel to one another. Most monocots have parallel venation. A blade of grass provides the best example of parallel venation.
Pinnate venation – many tropical monocots have large leaves in which secondary veins are parallel to one another, but they originate from a prominent midrib. Bananas, heliconias, and many arums exhibit pinnate venation.
Netted venation – most broad-leaved dicots have veins that branch repeatedly and are not oriented parallel to one another. Also called reticulate venation. There is much variation that is useful for distinguishing families.
Special venation patterns
Curvinerved as in Melastomataceae.
Looping and convergence near margins as in Myrtaceae and others,
Palmate as in Malvales (Bombaceae, Malvaceae, Sterculiaceae, Tiliaceae).
Node – point on a stem where leaves originate. The distance between successive leaves is, therefore, the internode.
Phyllotaxy – the arrangement of leaves in relation to the stem
Alternate – left-right-left-right arrangement of leaves along the stem. When internodes are short, it is sometimes difficult to discern alternate arrangements.
Opposite – leaves are positions directly opposite one another along the stem.
Whorled – three or more leaves originating from the same node.
Distichous – alternate leaves coming off the stem in two ranks in a well-defined left-right-left-right pattern and lying in the same plane.
Spiral – this is a form of alternate arrangement in which the leaves do not lie in the same plane. Do not confuse spirally arranged leaves having short internodes with whorled leaves. In spiral arrangements there is only one leaf per node.
Decussate – a special case of opposite leaf arrangement in which each pair of leaves is offset by 90° from the next pair of leaves. This is common in the Verbena and Mint Families.
There are many variations on leaf shape. Elliptical, ovate, and obovate are some of the most common.
Elliptical – like an elongated oval
Ovate – like an egg
Obovate – like an upside-down egg
MISCELLANEOUS TRAITS OF LEAVES AND STEMS
Anisophylly – having mature leaves of different sizes, as in Solanaceae.
Exudations – sap or latex that oozes slowly or flows freely from the cut surface of a leaf or stem. Commonly clear or white, but also yellow, orange, or red.
Hairs – many leaves have hairs or trichomes covering their surface. A hand lens often reveals that they have interesting shapes and colors. Stellate hairs are star shaped (really more like an asterisk). T-shaped hairs are also known as Malpighiaceous hairs after the family in which they are found.
Mucilage - slightly sticky often clear substance exudes from a crushed up or chewed leaf or petiole. Common in Malvaceae.
Odor - Crush up the leaves and smell them. Or tear off a leaf and immediately smell the broken petiole. Primitive (ranalean odor), anise, turpentine, unripe avocado, guava, eucalyptus, rank, sweet, and green bean are all possible odors that can be important in identifying tropical plants. Taste can be helpful as well, but is not recommended for beginners who do not know which species are poisonous!
Pellucid dots – transparent spots on leaves. Usually visible on the undersurface when held up to the light. They are often associated with glands that secrete odors.
Stipules - seemingly insignificant flaps of tissue located where the petiole meets the stem. They are usually tiny and usually fall off by the time the leaf has matured. The only evidence of their presence may be a tiny scar.
Strong bark or stringy bark – some bark can be peeled from the stem in long strips and is sometimes used as twine. Most bark, “weak bark”, will not peel in long strips, if at all. Common in Annonaceae and Malvaceae sensu lato.
Terminal - meaning a structure, such as a bud or stipule, located at the tip of a branch.