The study of leaf arrangements, or phyllotaxy, considers not only the descriptive classification of leaf arrangements but also theories regarding the cause of such arrangements.
The function of the arrangement of leaves (phyllotaxy) is to increase a plant’s ability to carry on photosynthesis by positioning the leaves in such away as tomaximize the surface area available to intercept sunlight. Leaves may be either caulescent (on obvious stems) or acaulescent (with no obvious stems).
Flowering plants have three basic types of arrangements: alternate spiral; opposite; and whorled or verticillate. The alternate spiral arrangement is generally considered to be the most primitive condition, with the opposite and whorled conditions being derived by suppression of internode development.
There are two major hypotheses regarding the processes governing these basic arrangements. The field hypothesis of phyllotaxy posits that, as leaf primordia (newleaf cells) are created by the plant, a zone that inhibits the growth of other primordia is laid down around it, and not until the shoot tip has grown beyond that zone can a new leaf primordium be laid down. The first available space hypothesis posits that new leaves grow as soon as the plant shoot has grown out far enough to allow space for them.
The various types of leaf arrangements are usually one of the easiest vegetative characteristics to use in helping to identify vascular plants. This is especially true when leaf arrangement is combined with other characteristics, such as the presence or absence of petioles or the quality of being sessile or nonsessile. Other characteristics include the shape of the leaves and the appearance of the margins, bases, and apices types.
Alternately arranged leaves produce one leaf per node. These leavesmay be on alternate sides of the stem (2-ranked or distichous), on one side of the stem (1-ranked or secund), or in a spiral around the stem. If 2-ranked leaves overlap, as in some oncidium orchids and iris species, then they are referred to as equitant.
Leaves of members of the grass family (Poaceae) are distichous and alternate. Their leaves differ from most other vascular plant leaves in that they normally consist of a split tubular sheath that surrounds the stem and more or less linear blades held at right angles to the stem. They also have a small, tonguelike structure (ligule) at the junction of the sheath and blade, although in some species it may be obsolete.
Spiral arrangements involve alternately arranged leaves in which each succeeding stem node and attached leaf is rotated slightly from the nodes below and above it. If the spiral is to the right, it is referred to as dextrorse; if to the left, it is referred to as sinistrorse.
When two leaves occur at one node, the arrangement is called opposite. Oppositely arranged leaves may be either 2-ranked, as in Mexican heather (Cuphea hyssopifolia) in the henna family (Lythraceae), or 4-ranked or decussate, in which each succeeding pair of leaves is at right angles to the pairs above and below them. Decussate arrangement of leaves is characteristic of the mint family (Lamiaceae), themaple family (Aceraceae), and some members of the milkweed family (Asclepiadaceae), such as Asclepias viridis.
Whorled or Verticillate
When three or more leaves occur at one node, a whorled or verticillate arrangement is produced. The genera Galium and Sherardia in the madder family (Rubiaceae) are characterized by whorled leaves, as is also Isotria in the orchid family (Orchidaceae).
Rosettes, often referred to as basal rosettes, occur in acaulescent plants, such as the common dandelion (Taraxacum officinalis) in the sunflower/aster family (Asteraceae). Acaulescent plants do have a stem, but the internodes are greatly contracted, and the leaves have a spiral alternate arrangement.
Many biennial plants, such as carrots (Daucus carota) and poison hemlock (Coniummaculatum) in the carrot family (Apiaceae), will produce a basal rosette during the first year of growth, followed by the production of a flowering stem with alternate leaves the second year.
A leaf or a pair of connately fused leaves with the stem going through the center are referred to as perfoliate. Montia perfoliata and Bupleurum rotundifolium are examples of the perfoliate condition derived from a single leaf.
Silphium perfoliatum is a good example of the basal connate fusion of leaves to achieve the perfoliate condition. The upper cauline leaves of henbit (Lamium amplexicaule) in the mint family (Lamiaceae) are sessile and clasping the stem but are not actually fused.
The leaves of most conifers have developed with the need to minimize water loss while maximizing photosynthesis under relatively cold and dry (physiological drought) conditions where water is often not easily obtained. Needle-like leaves arranged in close, regularly spaced secund (one-sided, like a comb) divisions are referred to as pectinate or comblike.
Acicular leaves arranged in bundles or fascicles are typical of pines and spruces. In pines each fascicle is composed of two, three, four, five, six, seven, or eight divisions of needles, which form a more or less cylindrical shape if pushed together. The fascicles are spirally arranged on the tree branches.
In eastern red cedar (Juniperus virginiana), the leaves are reduced to minute scales, which have an opposite decussate arrangement, giving the appearance of 4-ranks. The scales are imbricate or overlapping, much like shingles on a roof.
The leaves of the yew (Taxus) are sharp-pointed, flattened, and narrowly lance-shaped. They are spirally arranged on the branches but almost always give the appearance of being 2-ranked.
This is also true for the dawn redwood (Metasequoia) and bald cypress (Taxodium distichum), both of which are deciduous in the fall, dropping entire branchlets with the attached leaves. Yew podocarpus (Podocarpus macrophylla) in the podocarpus family (Podocarpaceae), on the other hand, has a obvious spiral arrangement of the leaves.