Leaves as sources of taxonomic characters
Floral characteristics are most often used in taxonomy, but sometimes vegetative features come in handy. Leaves are an important source of taxonomic characters, although variable (also roots and stems)
Leaf parts: base, petiole, blade (=lamina)
- Leaf base may have stipules (stinging nettle, Urtica has stipulate leaves) or not (hedge nettle, Stachys has estipulate leaves)
- Stipules are outgrowths of the leaf base. The leaf base grows before the rest of the leaf (development fro base to tip - acropetal development) and so stipules can be much larger in early stages of development than the blade. They often function in protection of the lamina and/or apical meristem. To determine stipule function, one needs to look at the leaf as it develops.
- Axillary buds are found in the axils of leaves.
- Leaves without petioles are referred to as sessile.
- Leaf parts can differ chemically as well as morphologically: petiole of rhubarb (Rheum) is edible,
blade is poisonous.
Types of compound leaves
- Simple leaves have one blade; compound leaves have two or more separated blades or leaflets.
- Pinnately compound (leaflets attached at different points) vs palmately compound (leaflets attached at the same point).
- Midrib of a pinnately compound leaf called a rachis, petiol is not part of the rachis
- Leaves can be once compound (pinnate), twice compound (bipinnate) or even thrice-compound (tripinnate)
- A leaf with 3 leaflets is called “trifoliate”: can be either pinnate or palmate.
- A pinnatifid (or palmatifid) leaf is almost compound. It is deeply lobed almost to the midrib but not quite, so separate leaflets are not present.
Leaf dissection is dependent on primary venation (lobation)
Acer (maple) – palminerved (palmately nerved) and palmatifid
Quercus (oak) – penninerved (pinnately nerved) and pinnatifid
Developmental mechanisms behind the compound leaf
- In compound leaf development KNOX is down regulated, then after a time it “comes back on”
- Compound leaf is more “stem-like” at the molecular level: re-expresses KNOTTED-LIKE HOMEOBOX (KNOX) genes at late-stage of organogenesis
KNOX-genes can be manipulated in Arabidopsis to show the origin of compound leaves
Wild type Arabidopsis – unlobed
KNOX over-expression in transgenic Arabidopsis - lobed
These clues about KNOX genes and lobbing come from basic research on Arabidopsis, but since it has simple
leaves there are some limitations as to what can be learned about the molecular basis of compound leaf development from this system. More pertinent are experiments with KNOX-genes and compound leaves Amborella (simple leaves) vs. Oxalis (trifoliate compound leaves). Oxalis leaves are seen to rexpress KNOX are later stages of development correlated with the formation of compound leaves.
Leaves exhibit great variety of patterns of grouping on the stem. This is putatively adaptive.
- Leaves are borne at specific points on stems called nodes
- One leaf per node - leaf arrangement = alternate
- two leaves per node - leaf arrangement = opposite
- Three or more leaves per node - leaf arrangement = whorled
- Leaf orientation is adaptive and is controlled by petiole growth
- Plants in hot, dry areas often have leaves oriented vertically to help prevent overheating and photobleaching of chlorophyll. For example, many Eucalyptus species have leaves that hang vertically (though as seedlings in the shade they may have horizontal leaves). This is a good example of plant structure that can be seen in terms of function and adaptation.
NOTE ON TELEOLOGY
It is tempting when talking about adaptation to use "teleological" forms of speech, i.e. the imputation of purpose. Teleology (from the Greek “telos” = purpose) is Aristotle’s “final cause” or the imputation of purpose to
the natural world. Adaptations have an effect, driven by selection, but not a purpose.
Teleological explanations for plant adaptation must be AVOIDED.
Leaf shapes: some common terms
- Most leaves are either elliptic, ovate (egg shaped - narrower toward the apex) or obovate (egg shaped but narrower toward the base
- Other shapes are: lanceolate (lance-shaped - much longer than wide, widening above the base, and tapering to the apex or oblanceolate (lance-shaped but wider near the apex and tapering to the base)
- Spatulate (spoon shaped)
- Peltate (umbrella shaped, with petiole attached within the margin of the blade)
- Reniform (kidney shaped)
Shapes of leaf apex and tip
- The shape of the leave base and tip may be important taxonomic characters. They may be acute (sharp angled, less than 90°,with straight sides) or obtuse (blunt angled, more than 90°)
- Special terms for the leaf tip: acuminate (tapering to an extended point with slightly concave sides), truncate (with the apex straight across, as if cut off), emarginate (with a shallow notch at the apex Liriodendron is strongly emarginate, i.e. deeply cleft at tip, caused by the leaf tips stopping growing while the rest of the leaf keeps growing)
- Special terms for the leaf base: cordate (heart shaped), auriculate (with earlike lobes), sagittate (arrowhead shaped, e.g. some Rumex).
Shapes of leaf margins
- Leaf margins may be smooth (entire), or variously toothed or lobed. If deeply divided they are called dissected.
- Dentate means with coarse, sharp teeth pointing perpendicular to the margin.
- Serrate means with sharp teeth pointing towards the apex.
- Crenate means with shallow, rounded teeth.
- Doubly serrate or biserrate means serrate but with the teeth also having teeth on them.
- Lobed means indented 1/4 to 1/2 way to the midrib. Lobes can be palmate, if they all connect toward the base of the blade, or pinnate, if they are along the length of the leaf.
Serrature is a general word for the pattern of leaf toothing. Note: angles = points of teeth; sinus = gap between teeth.
Leaves are a very plastic organ in evolution.
- Succulent leaves are thick and store water.
- Tendrils are twisting, threadlike structures by which plants attach to supports. They are usually either modified leaves, modified leaflets on a compound leaf (e.g. Pisum), or modified stipules (e,g. Smilax). (Caution they may also be modified stems as in Vitis).
- Spines - modified leaves for defence, e.g. cacti; BUT do not confuse with thorns (modified stems) or prickles (dermal products)
- Phyllodes are flattened, leaf like petioles, with the blade missing. E.g. Australian Acacia
- Bracts - reduced leaves in an inflorescence
- Carnivorous leaves: ascidiate (pitchers), glandular (fly paper) or traps (venus flytrap, bladder worts)
Hairs and surface features of plants
- Plant surfaces may be: glabrous (smooth), glaucous (with a whitish or blueish, waxy coating) or hairy.
- Hair types: pubescent (soft short hairs), ciliate (fringe of hairs), hirsute (long, shaggy hairs); hispid (stiff, rough hairs), scurfy (covered with scales), stellate hairy (with branched hairs), tomentose (densely matted soft hairs), villous (long, fine, soft hairs), urticate (with stinging hairs)
- Woolly hairs - insulate plants in deserts, high altitudes, or high latitudes.
- Glandular hairs – secretory. Protect from insects
Some terms describing surface texture
- Coriaceous - tough and leathery.
- Rugose - wrinkled.
- Scabrous - rough to the touch.
- Scarious - thin and membranous, usually dry and papery