Arboretum Images 6b

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Palomar College Arboretum Images 6b: Trees #2
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Olive Family (Oleaceae)

Chinese fringe tree (Chionanthus retusus), a beautiful tree in the olive family (Oleaceae) native to China. Other trees in the olive family within the Arboretum include the Arizona ash (Fraxinus velutina) and the evergreen ash (F. uhdei) native to Mexico.


Protea Family (Proteaceae)

Queensland nut (Macadamia integrifolia) showing leaves, fruit and fragrant flowers. The inner "nut" that is cracked open is actually a thick-walled seed. Contrary to numerous on-line and printed references, it is not the endocarp of a drupe. The dry outer husk splits open along one seam, exposing the large seed. This is characteristic of a fruit called a follicle.

  More Information About The Macadamia Nut  


Beech Family (Fagaceae)

Pin oak (Quercus palustris). This deciduous oak is native to the central and eastern United States. It is one of the ten oak species in the Palomar College Arboretum.

Bur oak (Quercus macrocarpa), another deciduous oak native to the eastern United States.

Southern live oak (Quercus virginiana), an evergreen oak native to the eastern United States. The fuzzy caterpillarlike galls on the underside of the leaf are caused by the gall wasp Andricus, possibly A. chrystallinus. This gall is common on California white oaks, including the blue oak (Q. douglasii), leather oak (Q. durata) and Oregon oak (Q. garryana). This wasp apparently also likes the sothern live oaks in the Palomar College Arboretum.

  See Article About Plant Galls  
See Blue Oaks Of California

Leaves and fruits (nuts) of the Japanese chinquapin (Castanopsis cuspidata), a genus related to oaks.


Moonseed Family (Menispermaceae)

Moonseed (Cocculus laurifolius), a small tree or shrub in the moonseed family (Menispermaceae) native to the Himalayas. The infamous South American vine curare (Chondodendron tomentosum) also belongs to this interesting plant family.

  See The South American Vine Called Curare  


Buckthorn Family (Rhamnaceae)

Jujube (Ziziphus jujuba), an Asian tree cultivated for its small brownish or rust-colored fruits (technically called drupes) which superficially resemble olives in general shape and structure. The fruits are picked when they reach their typical rust color, but must be left to wither for some time until the pulp (mesocarp) becomes spongy and sweet. The fruits are soothing to sore throats and are used to flavor certain medicines. In Asia this is one of the trees inhabited by the lac insect, the excretions of which are the source of shellac. A native species of jujube (Z. parryi) occurs in the desert mountains of San Diego and Imperial Counties, California..

  See The Ripe Fruits Of Jujube  


Casuarina Family (Casuarinaceae)

A. The stem and apical spore-bearing cone (strobilus) of a horsestail (Equisetum), a primitive non-flowering plant in the division Sphenophyta. B. The stem of a species of Casuarina, a flowering tree native to Australia. These unrelated plants belong to different divisions of the plant kingdom, and yet they both have jointed stems with whorls of scale-like reduced leaves at the nodes. This an example of homoplasy, a term that includes both parallel and convergent evolution. The Palomar College Arboretum has six species of Casuarina.

  See Convergent & Parallel Evolution  

Images of Casuarina Species in the Palomar College Arboretum

The genus Casuarina includes about 50 species of trees native to Australia and the South Pacific Islands. [Some species have now been placed in the genus Allocasuarina.] The generic name is derived from the Cassowary (Cassuarius), a large, flightless bird related to the Emu, in reference to the long, slender drooping branchlets that resemble the feathers of this bird (see link below). The slender branchlets also resemble the long needles of a pines and are the reason these trees are often called "Australian pines." Other common names include she-oak, beefwood and ironwood. The hard, dense wood is used for lumber and firewood. Casuarina species may be monoecious and dioecious, and they are wind pollinated. Female trees produce small woody "cones" composed of small bracteoles that separate at maturity releasing numerous flattened, one-seeded, winged fruits (samaras). The branchlets are composed of many nodes or joints, each with a whorl of minute scale leaves that are sometimes described as teeth. Some species can be separated by the number of leaf scales per node. It is indeed a challenge to identify the six species of Casuarina planted in the Palomar College Arboretum.

  See Hunters Carrying Dead Cassowary In New Guinea  

Note: What I have identified as Casuarina cunninghamiana in the following images could easily be C. equisetifolia. They are very similar species. In general, C. cunninghamiana has 8-10 leaf scales per node, while C. equisetifolia has 6-8 leaf scales. The branchlets I have examined typically have 9.

Left: A graceful Casuarina with drooping branchlets just north of the main entrance to the Arboretum. It has 10 scale leaves per node and is similar in some aspects to C. stricta. The 10 longitudinal ribs on the branchlets, that correspond with the 10 scale leaves, are minutely pubescent along the crest of the ribs. B. C. torulosa, a species with bluish-gray foliage and thick, corky bark. Some botanists place it in the genus Allocasuarina. It has very slender branchlets with only 4 minute leaf scales per node. Other species of Casuarina have 6-16 (18) leaf scales per node.

Left: A large Casuarina stricta south of the main Arboretum entrance. This species has long, drooping branchlets that superficially resemble the leguminous Jerusalem thorn (Parkinsonia aculeata). The branchlets are thicker than most other species of Casuarina in the Arboretum. This species has 10-12 leaf scales per node. Right: A thicket of Casuarina glauca on the upper slope of the Arboretum. This is a very invasive species with spreading root suckers. It has 15-16 leaf scales per node, compared with 8-10 for C. cunninghamiana and 6-8 forC. equisetifolia. The latter two species also have more slender branchlets when compared under a 10x hand lens. It is interesting to note that both C. cunninghamiana and C. glauca are dioecious. In addition, C. glauca has prolific root suckers from an extensive spreading root system and is an aggressive weed in the Hawaiian Islands. In fact, C. glauca apparently sucker sprouts much more commonly than C. equisetifolia in Hawaii. It also hybridizes with C. cunninghamiana.

Casuarina species are extremely difficult to identify. The following is my best attempt based upon notes from horticulturist James R. Kelly taken about 25 years ago. A. C. cristata, 9-10 leaf scales per node. B. C. stricta, 10-12 leaf scales per node. C. C. glauca, 15-16 leaf scales per node. D. C. torulosa, 4 leaf scales per node. E. C. equisetifolia, 6-8 (9) leaf scales per node. This latter species may be C. cunninghamiana.

Left: Casuarina glauca often misidentified as C. equisetifolia or C. cunninghamiana. It has 15-16 leaf scales per node and glaucous branchlets. Most references list 8-10 leaf scales per node for C. cunninghamiana compared with 14-18 leaf scales for C. glauca). C. equisetifolia typically has more slender branchlets with 6-8 leaf scales per node. Right: This appears to be C. torulosa. The slender branchlets have only 4 minute leaf scales per node. The only verified Casuarina species in the Arboretum with 4 leaf scales per node is C. torulosa.

Casuarina glauca. Left: Male (pollen-bearing) flowers. Right: Female (seed-bearing) flowers. Each flower cluster produces a woody, seed-bearing cone (top). The flowers are apetalous and the colorful threadlike clusters are actually receptive styles. Since the flowers are produced on separate male and female trees, this species is dioecious.

Close-up view of the stems of Casuarina glauca. In the Palomar College Arboretum it typically has 15-16 leaf scales per node. According to the Flora of Hawaii (1999), this species has 9-18 leaf scales per node, compared with 6-8 (9) for C. equisetifolia and 8-10 for C. cunninghamiana. It is a dioecious species with separate male and female trees in the population. It is also a prolific sucker-sprouter from invasive, spreading roots. It develops root nodules containing the symbiotic nitrogen-fixing bacteria (actinomycete) Frankia. Nitrogen fixation is undoubtedly one of the reasons this species can grow well in nitrogen-poor soils.

  Nitrogen Fixation In Root Nodules  

Casuarina glauca, showing whorls of 15-16 leaf scales at the nodes.

Large Casuarina cunninghamiana south of the main entrance to the Arboretum and close-up view of the cone-bearing branchlets. The sucker sprouts to the left of the tree are from a nearby C. glauca. Like C. equisetifolia, the branchlets are more slender than C. glauca. It has 9 leaf scales per node compared with 6-8 for C. equisetifolia. These traits can easily be observed with a 10x hand lens. It is possible that this tree is C. equisetifolia; however, it was originally identified as C. cunninghamiana by horticulturist James R. Kelly. The seed-bearing cones of all three species are very similar.

Seed cones of Casuarina. Left: C. cunninghamiana. Center: C. glauca. Right C. torulosa? Although they superficially resemble the cones of a pine tree, they are morphologically very different. They are produced by multiple, apetalous, female flowers, each enclosed by pairs of minute bractlets that enlarge into the woody valves of the cone. The cone-like structure is really a dry, multiple fruit that originated from an ovoid, multiflowered, head-like inflorescence.

A seed cone derived from the ovoid head-like inflorescence of Casuarina glauca composed of many pairs of woody valves that developed from floral bractlets. At maturity, each pair of valves separates, releasing a flattened, one-seeded, winged samara. Since each pair of valves represents a single flower, the entire cone-like structure is technically a dry, multiple fruit composed of many ripened ovaries (carpels) derived from many unisexual, apetalous female flowers. The samaras are technically not seeds because they came from ovaries (carpels) of female flowers and represent one-seeded, winged fruits.

The winged, one-seeded samaras of Casuarina glauca resemble seeds from a pine cone until you realize that they are only 5 mm (1/5th of an inch) in length. Technically, these are not the same as pine seeds because they came from small female flowers and represent a one-seeded, winged fruits.

The trunk of a Casuarina species (possibly C. stricta) in the Palomar College Arboretum. Although native to the south Pacific region, these trees are naturalized and planted throughout temperate and tropical regions of the world. Some species are called "ironwoods" because their dense, reddish-brown wood was used extensively by native Polynesian islanders.