GEOG/ES&P 330

California Ecosystems

Leaves

Familiarity with the different shapes, textures, and arrangements of leaves and stems is critical for use of a key to identify plants. In what follows, I'll provide an illustration of a leaf term from the Calphotos web site.

Leaf complexity

Leaves can be simple or compound. A simple leaf is borne singly on or along a stem, often attached with a stemlet called a petiole. An example you'll be meeting in Palos Verdes is Rhus integrifolia or lemonadeberry (and you can see the petioles at the base of these leaves).

A compound leaf is made up of a group of leaflets.

• Sometimes the leaflets all come out of a common origin point. This arrangement of leaflets is palmately compound. An example is Lupinus longifolius, or longleaf bush lupine (which some of you may have encountered in Lab 1).
• Other times, the leaflets are arranged in pairs running along a "stem," which is called a rachis. Almost always, there will be a single leaflet at the very end of the rachis. An example is California wild rose

It can be hard to decide whether you're looking at a true stem with a lot of simple leaves along it or a compound leaf running along a rachis.

• Compound leaves have a single leaflet at the end of the rachis, while true stems will have, instead, a growth bud with leaves emerging from it.
• A simple leaf will often have a bud at its axil, or the angle between the leaf and the stem (sometimes a whole new twig will grow out from the axil); compound leaves may have a bud at the base of the petiole below the rachis, but you will never see a bud at the base of a leaflet.

Leaf attachment

Leaves may attach to stems in a variety of ways.

• Leaves, whether simple or compound, may attach to their stems through a small "stemlet" or stalk, which is called a petiole. An example is Rhus ovata or sugarbush, which is a close relative of lemonadeberry.
• A pinnately compound leaf has a central axis that looks like a stem, to which the leaflets attach. This is called a rachis. Here is Rubus ursinus or California blackberry.
• An axil is the angle between a leaf or its petiole at the node where it commects with the stem.
• An axillary bud is a growth center, from which a new leaf or stem or flower or inflorescence will emerge. An example is Styrax officinalis or California snowdrop bush. The axillary bud example is kind of hard to see here, but it's about halfway down in the right center. This plant, by the way, is found in a disjunct distribution, in California and in the Mediterranean, but it is believed to be native in both places (not an introduced exotic in one)!
• A stipule is a small structure at the base of a petiole, usually found in pairs, which is distinct from an axillary bud, in that it will not give rise to a new leaf, stem, or flower. They may look like tiny dots or scales, miniature leaves, spines, or wings running along the petiole. This is Lathyrus vestitus or Pacific pea
• Sessile describes a leaf that attaches directly to a stem without a petiole of any sort. An example is Diplacus aurantiacus or sticky monkeyflower.

Leaf margins

• Entire: no teeth or lobes, though the margins may be wavy. Rhus ovata or sugarbush
• Lobed: leaf margines are cut or indented, sometimes pretty deeply. Quercus lobata or valley oak or roble
• Toothed: leaf margins are lined with small irregularities, whether rounded or sharp.
  • Crenulate: margins are lined with small, rounded teeth. Salvia leucophylla or purple sage
  • Dentate: lined with sharp teeth running roughly at right angles to the leaf margin. Penstemon palmeri or Palmer's penstemon
  • Serrate: lined with sharp teeth pointing forward toward the tip like a saw. Brickellia californica or California bricklebush
  • Spinose: teeth sharpened to a spiny point. Hazardia squarrosa or sawtooth goldenbush

Leaf and leaflet shape

• Linear: much thinner than long. Salix exigua or narrowleaf willow
  • Needle/needle-like: linear but with a thick or rounded cross-section Pinus monophylla or piñon
  • Filiform: linear to the point of thread-like. Artemisia californica or California sagebrush
• Lanceolate: much longer than wide, but not so much as linear; often a bit wider at the base. Baccharis salicifolia or mulefat
• Oblanceolate: like lanceolate, except the widest part is toward the tip.Eriogonum fasciculatum or California buckwheat
• Spatulate: much wider at the tip than the base. Sesuvium verrucosum or sea purslane
• Elliptical: a symmetrical oval, with the widest part in the middle.Heteromeles arbutifolia or toyon
• Oblong: elliptical but with roughly parallel sides in the middle, a flattened oval.Malosma laurina or laurel sumac
• Ovate: elliptical but wider toward the base, like an egg. Rhus integrifolia or lemonadeberry
• Obovate: elliptical but wider toward the tip. Frankenia salina or alkali heath
• Orbicular: almost perfectly round. Arctostaphylos parryana or Parry manzanita
• Reniform: kidney-shaped, noticeably wider than long. Abronia latifolia or coastal sand verbena
• Cordate: heart-shaped, with a notch at the rounded base where the petiole attaches or where the leaf clasps the stem. Keckiella cordifolia or heart leaved penstemon
• Obcordate: heart-shaped, but with a notch at the wide, rounded tip of the leaf. Oxalis trilliifolia or three-leaf woodsorrel
• Deltoid: triangular, with the base wide and the tip narrow. Brickellia californica or California brickelbush
• Cuneate: also triangular, but with the tip wide and the base narrow. Cercocarpus betuloides or mountain mahogany
• Rhomboidal: roughly diamond-shaped, with the widest part in the middle. Encelia californica or California encelia
• Falcate: sickle-shaped, a narrow leaf with a crooked or curving axis. Allium falcifolium or scytheleaf onion (Northern California)
• Lobed: margins indented but not all the way to the axis so that the leaf is not actually divided into leaflets like a compound leaf is:
  • Palmate: lobed like a hand and its fingers. Acer macrophyllum or bigleaf maple
  • Pinnatisect: lobed perpendicularly to the axis. Descurainia pinnata or tansyleaf mustard
  • Hastate: three-lobed, with the two basal lobes more or less perpendicular to the axis. The leaf is triangular in outline. Calystegia macrostegia or island false bindweed or
  • Sagittate: three lobed, with the two basal lobes barbed back over the petiole, almost parallel to it. The leaf looks like an arrowhead or spearhead. Calystegia purpurata or morning glory

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Document maintained by Dr. Rodrigue
First placed on web: 02/02/16
Last revision: 09/15/21