Vegetative Terminology (Part 2)
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Economic Plant Illustrations #30

Botany 115 Vegetative Terminology

Modified Roots, Stems and Leaves (Part 2)

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Thorns & Spines: Modified Stems & Leaves

A. Thorn of Pyracantha. B. Spine of barberry (Berberis). The thorn is technically a modified, sharp-pointed stem. It occurs in the axil of a leaf where a branch would normally develop. The spine is technically a modified, sharp-pointed leaf. Since it has a bud in its axil, the spine occurs in the relative position of a leaf. Some spines are called "stipular spines" because they are modified, sharp-pointed stipules at the base of a leaf. The swollen-thorn acacias of Central America actually have enlarged, hollowed-out, stipular spines occupied by stinging ants.

Thorns of the honey locust (Gleditsia triacanthos), a shade tree native to the eastern United States. The sharp-pointed thorns arise in the axils of the compound leaves (left) and from the main trunk (right). Climbing this tree can be hazardous to your health.

Crucifixion thorn (Castela emoryi), a rare tree in the quassia family (Simaroubaceae) native to the Colorado Desert of Imperial County, California and adjacent Arizona. The branchlets are stiff and very sharp. Although it appears leafless most of the year, it does produce small, scalelike, ephemeral leaves during the spring growing season. This family also includes the tree of heaven (Ailanthus altissima), a naturalized tree from China that was introduced into the western United States by early settlers.

A. Prickles of cat's claw acacia (Senegalia greggii). B. Stipular spines of sweet acacia (Vachellia farnesiana var. farnesiana). Unlike stipular spines at the bases of leaves, prickles arise from the cortex and epidermis of plant stems. The classic thorns of roses are actually prickles.

Trunk of the floss silk tree (Chorisia speciosa), a tropical American tree in the kapok family (Bombacaceae = Malvaceae). The sharp-pointed, woody outgrowths from the trunk are often called thorns; however, they are technically referred to as prickles.

See Cotton-Filled Fruits Of The Floss Silk Tree

Two examples of true spines (modified leaves). Left: Barberry (Berberis vulgaris) showing lateral buds arising in the axils of 3-pronged, modified leaves called spines. Right: Gum tragacanth, an Iranian locoweed (Astragalus), showing the rigid, sharp-pointed leaf rachises after the leaflets have dropped off. The natural polysaccharide thickening agent called gum tragacanth is obtained from the sap of this plant.

Left: Canary Island date palm (Phoenix canariensis) native to the Canary Islands. Right: Cretan date palm (P. theophasti) native to Greece and Turkey. The latter species is similar in appearance to the cultivated date palm (P. dactylifera). The basal leaf segments (pinnae) are modified into stiff pines. Like sharp-pointed daggers, they are a hazard when climbing into date palms to pick the fruit.

Stipules and Stipular Spines

Left: A simple leaf in which the blade is not divided into leaflets. Stipules are small, paired appendages at the base of the petiole. Right: A compound leaf in which the blade is divided into leaflets. This is an odd pinnate leaf because it has a single terminal leaflet. In some species, such as black locust (Robinia pseudoacacia), the stipules at the base of the petiole are modified into a pair of sharp spines.

Stipular The Spines Of Swollen-Thorn Acacias
See The Wayne's Word Article About Acacias
Photos and Information About Gum Tragacanth

Three species of Acacia with swollen stipular spines that are hollowed out and occupied by symbiotic ants. Left: The bullhorn acacia (Acacia cornigera), a swollen-thorn acacia native to Mexico and Central America. In its native habitat, colonies of stinging ants (Pseudomyrmex ferruginea) occupy the hollowed-out thorns and fiercely defend the tree against ravaging insects, browsing mammals and epiphytic vines. In return, the host supplies its little guardian ants with protein-lipid Beltian bodies from its leaflet tips (yellowish granules in photo) and carbohydrate-rich nectar from glands on its petiole (just above the pair of spines). Center: Another Central American swollen thorn acacia (A. collinsii) with an acacia ant (P. ferruginea) sipping nectar from the petiolar nectary. Right: The African whistling thorn acacia (A. drepanolobium). The common name comes from the whistling sound that is produced when wind blows across the large hollowed-out thorns. Since the "thorns" on these trees are technically pairs of modified stipules, they are more correctly referred to as stipular spines.

See The Wayne's Word Article About Acacias
See Stipular Spines Of Swollen-Thorn Acacias
Necklace & Seed Doll Made Of Acacia Spines

Spines From Involucral Bracts (Phyllaries)

Star thistle (Centaurea solstitialis), a European annual of the sunflower family (Asteraceae) that has become a troublesome naturalized weed in the Central Valley of California. The involucre is composed of overlapping bracts (phyllaries), each tipped with a slender spine. The dry, stiff spines can readily penetrate skin and can puncture bicycle tires. Another yellow-flowered species with shorter spines (C. melitensis) is naturalized in disturbed areas of southern California. A purple-flowered perennial species without involucral spines is called Russian knapweed (C. repens). The latter species is listed under the genus Acroptilon in the Jepson Flora of California, 1993. Although the genus includes some of the worst weeds in California's pasturelands, it also includes some popular garden ornamentals, such as "dusty miller" (C. cineraria).

Spines From Modified Petioles

A primary leaf arising from the stem of cirio or boojum tree (Fouquieria (Idria) columnaris), a xerophytic shrub native to the Baja California peninsula. It belongs to Fouquieriaceae, along with ocotillo (F. splendens). After the blade falls off the spine develops into a woody, rigid, very sharp spine. Cuttings from ocotillo are sometimes planted for an impenetrable living fence.

Two types of leaves in the boojum tree (Idria columnaris). Left: Primary leaves develop on new growth and become petiolar spines after the blades drop off. Right: Secondary leaves arise from the axils of the spines (from primary leaves) when sufficient water is available. Idria and ocotillo are drought deciduous, and shed their secondary leaves during the dry season.

Left: Boojum tree (Fouquieria columnaris), also known as Idria columnaris. Right: Ocotillo (F. splendens).

Boojum tree (Fouquieria columnaris) in full bloom. [This Baja California endemic is also known as Idria columnaris.] The cream-colored corolla is sympetalous (with fused petals). The fragrant flowers are visited by bees, wasps and hummingbirds (see photo inset).

Hummingbird with its bill inserted into corolla of Boojum tree (Fouquieria columnaris).

An Ocotillo Look-Alike On Madagascar

Alluaudia procera, a member of the Didiereaceae. This spiny xerophytic shrub or small tree with small, drought deciduous leaves is native to southern Madagascar. Technically, the paired leaves have thorns rather than spines in their axils. It superficially resembles our spiny ocotillos (Fouquieriaceae) of Baja California, although the two families are not closely related. They both conserve moisture during periods of drought by shedding their small leaves.

Spines From The Cactus Family (Cactaceae)

Two types of Opuntia showing the jointed stem sections covered with spines. In cholla cactus (left) the stem segments are cylindrical. In prickly-pear cactus (right) the stem segments (called pads) are flattened.

A: The leafy cactus (Pereskia) showing a cluster of spines arising from the leaf axil. The spines are homologous (similar in structure & origin) to bud scales of an axillary bud. Although they arise in a leaf axil, they are not homologous to a stem; therefore, they are not called thorns. Some botanists refer to them as modified leaves. B: A cholla cactus (Opuntia) showing a cluster of spines arising in the axil of a small caducous (early deciduous) leaf. The spines develop from a meristematic growth center called the areole. In cholla cactus the areole and spine cluster occur on a raised, domelike or elongate area called the tubercle.

Opuntia subulata, an interesting species native to Equador. Clusters of
needle-like spines arise in the axils of fleshy, subulate leaves (red arrow).

Left: Jumping cholla (Cylindropuntia bigelovii). The cylindrical stem segments become detached readily, and serve to disperse and propagate this species asexually. Vegetative propagation by cloning is a good adaptation since populations of this cactus are often sterile. Center: Buckhorn cholla (C. ganderi), another Cylindropuntia with cylindrical stem segments. The spine clusters arise from ribbed, raised areas along the stem called tubercles (red arrow). Right: Opuntia chlorotica, a prickly pear Opuntia with flattened stem segments. This interesting southern California species is often called "pancake pear."

Buckhorn cholla (Cylindropuntia acanthocarpa) showing riblike tubercles that are elengate and slenter, more than 4 x longer than wide (2-3 cm in length). The stamen filaments are red in this species of cholla.

Jumping Cholla: The Most Painful Hitchhiker

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