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Retrophyllum, C.N. Page  1989

Retrophyllum description


Evergreen trees and shrubs, sometimes with swollen, fluted, or buttressed trunks. Bark fibrous, peeling in short strips and often becoming strongly ridged and furrowed with age. Crown broadly dome-shaped and spreading, consisting initially of regularly alternating pairs of branches. Branchlets variably differentiated into long and short shoots, these generally similar or the long shoots with scale leaves, the short shoots frondlike, flattened, and shed intact together with their leaves. Both shoot types hairless, remaining green for at least the first year, prominently grooved between the attached leaf bases. Resting buds naked, not very evident. Foliage leaves of horizontal shoots in pairs, usually (except in Retrophyllum minus) brought into a single plane by twisting of the petioles and of the twigs between the leaves, those to the right of the twig all with their upper surface facing up while those on the left have their lower surface showing (hence the scientific name, Latin for “backward leaf”), more or less sword-shaped.

Plants dioecious. Pollen cones cylindrical, single or in clusters of two to five at the ends of short, axillary scaly stalks or of leafy short shoots. Each cone with a few alternating pairs of bracts at the base and numerous, densely crowded pollen scales, each bearing two pollen sacs. Pollen grains medium (body 35-40 µm long, 50-60 µm overall), with two round air bladders that are smaller than the body, body and bladders relatively smooth externally, the bladders also with coarse internal sculpturing. Seed cones single or in pairs on short, scaly stalks in the axils of foliage leaves. Individual cones highly modified and reduced, with a single, plump, unwinged seed embedded in an inverted position in each of one or two fleshy, green to red or blue-black seed scales (the epimatium) in the axils of somewhat leaflike bracts that may become slightly fleshy but do not enlarge into a conspicuous, colorful, berrylike podocarpium, maturing and falling in a single season. Seeds usually with a crest along one side and over the tip. Cotyledons two, each with two veins. Chromosome base number x = 10.

Wood soft, light, sweetly fragrant, light brown, with a core of yellowish brown to reddish brown heartwood. Grain fine and even, with growth rings delimited by narrow bands of denser latewood. Resin canals absent.

Both leaf faces with interrupted lines of stomates not organized into distinct stomatal bands. Each stomate sunken beneath and largely hidden by the four (to six) surrounding inner subsidiary cells, which rise around the opening in a prominent, continuous Florin ring that is surrounded by a sunken moat. Leaf cross section with a single midvein flanked by wings of transfusion and accessory transfusion tissue extending up to halfway to the margin and with one to three (to five) resin canals beneath the midvein and none to three additional ones on each side extending the length of the leaf halfway between the upper and lower surfaces. Photosynthetic tissue of adult foliage leaves forming a palisade layer one or more cells thick covering both surfaces beneath the epidermis and adjacent discontinuous patches (between the stomates) of hypodermal cells.

Five species in the southwestern Pacific from Fiji and New Caledonia to the Moluccas and in western South America. This small group of species was long included as a distinctive botanical section within the broadly defined Podocarpus that was current until 1969. In that year, de Laubenfels included them as a section within a separate genus (originally Decussocarpus, later replaced with Nageia) that also included sections Nageia and Afrocarpus (here all recognized as separate genera). All three sections usually lack the united, swollen bracts of the seed cone that make up the podocarpium that gives Podocarpus its name. They also share leaves attached in pairs on opposite sides of the twigs, in contrast to the individually spirally arranged leaves of Podocarpus species in the restricted sense generally accepted today. Finally, Page treated each of these sections as a separate genus, based on differences in leaf structure and base chromosome numbers. This separation is adopted here because of the coherence and ready recognizability of these three groups, but recognition as separate sections within a single genus would also be justifiable taxonomically. This is because the three groups are mutually each other’s closest relatives, as shown by several DNA studies involving different genes, while the species within each group have likewise been shown to be closest to other members of the same group. Since each group is thus monophyletic, as are all three groups taken together, it is a matter of preference rather than scientific necessity whether they are recognized as separate genera or sections within a single genus. The various DNA studies also show that Retrophyllum, Nageia, and Afrocarpus are most closely related to Podocarpus, so their historical inclusion within that genus was not simply mistaken, even though relationships are better expressed by separating them.

The distinctive shoots of Retrophyllum, with the uniformly paired leaves each with a single midvein and all facing up on one side of the twig and down on the other, are unique among living conifers. Similar fossils can be traced as far back as the Eocene, more than 35 million years ago, in southern South America, New Zealand, and across southern Australia, but some of these may represent extinct genera since they differ in important details of the leaf and epidermal structure from any living species. Unlike some other southern conifer genera with species in both South America and Australia in the past and present, such as Libocedrus or Prumnopitys, no species of Retrophyllum are found today in New Zealand or eastern Australia. Species of Retrophyllum are not in general cultivation, and no cultivar selection has taken place.




Attribution from: Conifers Garden