The leaf is a lateral line organ springing from the axis of the shoot; it is generally flat and its growth is limited. The leaves are essential tools for collecting sunlight to use in the process of photosynthesis and· in emersed aquatic plant growth they regulate the loss of water vapor. They also have other functions such as food storage, and they may form reproductive cells.
Phyllotaxis is the name given to the arrangement of leaves in the nodes of the shoot (Ill. 1).
On an elongated shoot axis the leaf arrangement shows an alternate pattern (Ill.1.1).· Each node develops one leaf and the angle between the leaves of every two nodes is always specific, often two fifths of a plant's circumference. This alternate leaf arrangent is rare in aquatic plants, but it does occur, e.g. Lobelia cardinalis.
In two-rowed, alternate leaf arrangement (Ill.1.2) the angle between leaves of each two nodes is always 180°. The leaves are positioned on the shoot axis in two exactly opposite longitudinal lines as with· Potamogeton species.
Many aquatic plants have a decussate and opposite leaf arrangement (Ill.1.3); each node produces two leaves, opposite to each other and forming an angle of 90° with the leaves of neighbouring nodes. Examples for a decussate and opposite leaf arrangement are found in the genera Hygrophila and Ludwigia.
If three or more leaves develop from one node they form a whorl (Ill.1.4), e.g. genus Myriophyllum and species of Limnophila.
On a compressed shoot axis the leaves are positioned close to each other; they are arranged like rosettes (Ill.1.5).
The most common leaf , the foliage leaf occurs in a large variety of shapes and sizes. It· is composed of three main parts: the leaf-blade or lamina; the stalk or petiole; the leaf base or sheath (Ill.2). The lamina is the actual leaf area and can have many different shapes (Ill.3).
Apart from a characteristic outline, the lamina tips, edges and bases display marked differences and there are also variations in structure. A lamina may be tender and transparent; tough and flattened; wavy or bullate. If a lamina is wavy or bullate the upper and underside of the blade displays protrusions (usually fairly dense), e.g. Cryptocoryne aponogetifolia. There are also variations in coloration on the upper and undersides of the lamina. The leaf is traversed by a network of veins which encompass the vascular bundles.
In the group of plants possessing only one cotyledon a distinct midrib has parallel pronounced, longitudinal veins running on either side. In dicotyledonous plants, transverse veins branch off from the midrib. The length of the petiole can be very variable. It may be missing altogether in the leaves of some aquatic plants or, if the leaves are elongated and tapering at the base, the petiole may be just perceptible. A leaf without a petiole is often referred to as a sessile leaf. On the other hand, the leaf-base may be of minor importance and appear as an elongation and broadening of the petiole.
In some marsh and aquatic plants the leaf-base is so distinctly sheath-like in form that it is not always as discernible from the petiole as in Cryptocoryne species. Such sheaths also fulfil another function; they enclose and protect the succeeding young leaves for a time. The peltiphyllum is a special type of foliage leaf; its petiole is positioned approximately in the middle of the leaf underside. Examples for this type of leaf are seen in some Hydrocotyle species.
Some leaves are modified and deviate from the characteristic foliage leaf, e.g. scale leaves—often found on runners; bracts positioned near inflorescences; and—finally—the flower petals which are really transformed and modified leaves.
There is a definite link between the habitat of a plant and the structure of its foliage leaves. If an environment offers stable water conditions the leaves are usually composed of an upper epidermis, palisade and spongy tissue, and a lower epidermis. Dense, longish cells constitute the palisade tissue; the spongy tissue has a loose and porous structure. The cells of the spongy tissue are irregular in shape, and the tissue contains cavities. Chlorophyll is not present in either the upper or the lower epidermis; the lower epidermis possesses some localised stomata. Some evidence of this structure is noticeable in floating leaves (although here the stomata are embedded in the upper epidermis) (Ill. 4.1.) and the water leaves of those plants which grow primarily in marshes (Ill. 4.4.).
Typical water leaves do not possess any stomata, but the cells of their epidermis layers contain grains of chlorophyll. The water leaves are thinner; their inner structure does not possess palisade and sponge parenchyma which gradually disappear in a metamorphosis to a simpler state.· Only the two epidermis layers remain, e.g. the wa ter-weed family (Ill. 4.8) but develop air spaces (Ill. 4.2, 4.3, 4.6. and 4.7).
Many marsh plants adapt well to a submerged habitat; it is characteristic for them that they are heterophyllous, meaning they are able to bear leaves of different forms depending on how they grow, i.e developing submerged, floating and aerial leaves in certain conditions. For example, Sagittaria species often develop slender, linear submerged leaves, but their floating and aerial leaves are distinctly divided into petiole and blade (Ill. 5). The leaves of many dicotyledons tend to become pointed and feathery when floating; Limnophila aquatica is a good example. Occasionally, plants produce leaves with different blades if they grow submerged, e.g. Echinodorus berteroi and Hygrophila difformis.
Footnotes / Definitions
Decussate: arranged on a stem in opposite pairs at right angles to those above or below, resulting in four vertical rows.
Bullate: having a puckered or blistered appearance.
Cotyledon: a leaf of the embryo of a seed plant, which upon germination either remains in the seed or emerges, enlarges, and becomes green. Also called seed leaf.
Dicotyledon: a flowering plant with two embryonic seed leaves or cotyledons that usually appear at germination.
Palisade: Elongated, tightly packed cells in the upper region of the mesophyll of the leaf.· This is where most of the trapping of light energy occurs by the chloroplasts
Lamina: the expanded portion or blade of a leaf.
Leaf apex: the tip of a leaf opposite to the petiole.
Midrib: the the central vein of a leaf· it is usually continuous with the petiole.
Vein: the vascular structures on a leaf which· arrangement is called venation.
Petiole: the stalk or stem that connects the leaf to the plant, frequently with a basal enlargement called pulvinus (petiole are absent in sessile leaves).
Axil: the angle between the upper side of the stem and a leaf or petiole.
Stem: (also called the axis) from which the leaves arise.
Stipule: the pair of small, appendages one on each side at the base of the petiole.
Pinna: division of a usually pinnately divided leaf
The Complete Guide to Water Plants - Muhlberg