The transition from the underground parts of the plant to its above-ground parts is known as the “caudex” or “stock.”
Stem – the main supporting structure of the above-ground part of the plant. It bears leaves, buds, fruit and flowers. They are usually circular, although square sections can be found in certain plants. Stems can be either winged or possess other forms of outgrowth. They may be erect or horizontal. They are occasionally erect near the base and then arch in such a way that the tips are pendulous.
Woody plants are rarely referred to as having “stems.” Instead, words such as “branch”, “twig”, “shoot” or “trunk” are used. Which term is used depends on the question of the size of the trunk. Stems bear a bud or growing point at the tip and put forth long growth by means of this, bearing leaves, and so on. Stems have leafless parts of the stem in between. So plants have what is called a “stem-dimorphism”, in which some stems tend to bear active leaves that are hardly elongated at all. These form condensed short-shoots.
Leaves – most plants have leaves. Leaves contain some of the most important marks for plant identification. They usually occur directly on the stems, branches or twigs, although there are some cases in which they are borne in basal rosettes or on short-shoots.
The point on a stem, shoot or twig at which a leaf, pair of leaves or a whorl of them is borne is a “node.” Leafless parts of a shoot between the nodes are predictably referred to as “internodes.”
Leaves that last for only a single growing season are known as “deciduous.” They are usually thin, papery or similar to parchment in texture. Leaves that last for several seasons are known as “evergreen” leaves. Evergreen leaves are usually thicker, leathery or needle-like. Some species have leaves that are half-evergreen. In such cases, some leaves fall after one season,k whereas the others remain for a longer duration.
The place at which leaves are attached to stems are known as nodes. Sometimes there is a single leaf at each note. At other times, a pair (on opposite sides of the stem); at yet other times, a whorl.
Spirally arranged – there is a single leaf per node and leaves are arranged along a spiral.
Alternate/distichous – Leaves are similarly on one node but successive leaves exist on opposite sides of the stem.
Opposite – If there are two leaves at each node, they are usually arranged on opposite sides of the them.
Opposite and decusate – successive pairs of leaves are at a 90 degree angle to one another.
Whorled – several leaves at each node. Some plants have false whorls. Make sure to distinguish between them, caused by spirally arranged leaves borne close together.
Petiole – the stalk that attaches the node to the leaf. Leaves without petioles are described as “sessile.”
Axil – the upper angle between the petiole and the stem to which it is attached. Each axil has a bud or branch developed from a budg. Some plants have many buds in each axil.
Leaflets – leaves divided up into separate segments.
Simple – Undivided leaves, known also as “simple” and “entire”, unless they have lobing or toothing.
Toothed leaves – leaves with slightly incised margins. They are sharp and look like teeth.
Serrate – when a toothed leave’s toothing is sharp and regular.
Biserrate – when each tooth is itself irregularly toothed.
Dentate – when the teeth are larger and irregular. They are either regularly or irregularly so.
Compound leaves – leaves divided up to the midrib into separate leaflets. THey are made up of two or more separate segments called “leaflets”, which are sometimes stalked. Stalked leaflets are known as “petiolate.”
Crenate – the way the margin is described when the teeth are rounded instead of sharp.
Two ways in which leaves can be divided into leaflets:
1) the leaves are arranged like fathers, such that the leaflets are arranged parallel to one another along the sides of the main axis(rachis); this is simply a continuation of the petiole. This is known as “pinnate.”
2) All the leaves arise from the same point as the top of the petiole. This is known as “palmate.”
Trifoliate – a plant whose leaves consist of three leaflets.
Pinnatifid – Leaves that are lobed pinnately from one to two thirds of the distance from margin to midrib.
Palmatifid/palmatisect – leaves that are lobed in a manner reminiscent of palmate division.
Leaflets can themselves be toothed or lobed. Some are even divided into further leaflets. This phenomenon is commonly found in pinnate leaves, which are in turn known as bipinnate, or doubly pinnate, tripinnate, quadripinnate, or so on.
Stipules – the outgrowth the point at which the petiole or leaf attaches tot eh stem. These are very important to plant identification. They can be separate to the petiole or joined to it. The stipules of opposite leaves sometimes form a pair on either side of the stem. They exist in between the attached leaves’ bases. Some stipulates are deciduous. Leaves can be examined to determine whether or not they were part of a tree with stipules because the leaf will have visible scars where the stipules had been.
Leaf scars – Woody plants develop scars when leaves fall. Shape, position, and form of scars can aid in identification.
Veins – network of harder tissue within leaves, which provide mechanical support for the leaf. They also carry water-bearing and food-bearing tissues(xylem and phloem). They work with other structures, and together form what are known as “vascular bundles.”
Some leaves have veins in which there are many equivalent veins entering into the same leaf from the petiole or the base, running independently to the margins. These are known as “parallel” veins and are typically interconnected with other veinlets.
Reticulate- leaves in which there is a prominent midrib, which enters its leaf from the petiole, running up its median line to the tip, by which it gives off secondary branches and are themselves branched into a network are known as “reticulate.” Reticulate veins are found in leaves which grow to their final sizes around the margins. Parallel veins, however, are found in leaves which grow primarily at the base. It is difficult, if not impossible, to visually detect venation in thick or fleshly leaves.
Ptyxis – Young leaves which are compressed in such a way that they pack in vegetative buds. Most leaves under this label have its sides folded upwards along its midrib, as well as having its sides parallel and close together. This is known as “conduplicate.”
Conduplicate – having leaves whose sides are folded upwards along its midrib and its sides parallel and close together.
Plicate – Larger leaves that are lobed or divided, and in which each leaflet or lobe can be folded in such a way as to produce a “pleated” effect.
Supervolute – Leaves which are rolled up into a tube in its bud, having one exterior margin and another interior margin.
Involute – leaves which have their margins rolled downwards and under.
Circinate – Leaves which are elongate and rolled from the tip to its base; the upper surface is either inside or outside the spiral.
Adnate – When the stipule is joined to the petiole.
Connate – when organs of the same type are joined together, as in the case of the petiole and the stipule, noted above.
Pinnate leaves end up either in a single terminal leaflet or there might not be any obvious terminal leaflet. In the first case, we have an imparipinnate leaflet and in the second case there is a paripinnate.
Leaflets are not individual leaves. Leaves have a bud at the base of the leaf, within the axil, whereas leaflets have neither of these organs.
Cullen, J., and J. Cullen. Practical plant identification: including a key to native and cultivated flowering plants in north temperate regions. Cambridge, UK: Cambridge University Press, 2006. Print.