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Acmopyle, Pilger 1903

Acmopyle description


Small to medium, evergreen trees. Trunk usually single. Thin, fibrous bark, initially smooth but flaking in scales over time. Crown conical at first, becoming rather open with age. Branchlets weakly differentiated into long and short shoots, the former constituting the main axes of growth and bearing scale leaves, the latter bearing foliage leaves, green for at least the first year, hairless, prominently grooved between the elongate, attached leaf bases. Resting buds ill-defined, consisting of a loose aggregate of green scale leaves. Leaves spirally inserted, predominantly of two distinct forms, scale leaves and foliage leaves. Scale leaves found on long shoots, reproductive axes, resting buds, and at the base of short shoots, triangular, flattened top to bottom in the ordinary way. Foliage leaves found on short shoots needlelike, longest near the middle and much shorter near the beginning and end of each shoot, extending in a single plane on either side of the twig through bending of the leaf at its point of attachment, then running down onto the twig without a petiole. Needles standing out from the twig in a forwardly directed, shallow S shape, flattened side to side rather than top to bottom.

Plants dioecious. Pollen cones on simple or sparingly branched reproductive branchlets emerging from the tips of short shoots or along the length of long shoots that are densely clothed with scale leaves. Simple shoots bearing one or two pollen cones at their tips and the branched ones bearing a varied number of such single or paired pollen cones. Each cone cylindrical, with a few small bracts at the base and numerous, densely spirally arranged pollen scales, each scale bearing two pollen sacs. Pollen grains medium (body 40-50 µm long), with two (rarely three) somewhat kidney-shaped air bladders that are smaller than the ovoid body and diverge at about 100° away from the germination furrow, fairly smooth but with coarse internal sculpturing, the cap of the body thicker and more coarsely sculptured than the furrow side. Side cones single or two or three from the ends of short shoots, each at the tip of a separate densely scaly stalk, highly modified and reduced, without free bracts at the base and with (four or) five to eight bracts united with the cone axis and becoming swollen and fleshy and covered with warts (as a structure called a podocarpium), maturing in (1-)2 years. All of the bracts except the uppermost one with at least a remnant of a seed scale, one or two of the bracts each with a single, plump, unwinged seed that is united for about half of its length with the fleshy seed scale (a structure called an epimatium), the edge of the epimatium marked by a distinct ridge around the seed. Seed with the fleshy layer of the seed coat and epimatium surrounding an extremely hard and thick inner shell, the ovule opening upward or slightly to the side (hence the scientific name, Greek for “tip gate”). Cotyledons two, each with two veins. Chromosome base number x = 10.

Wood soft, light, not fragrant, yellowish brown to light brown, with little distinction between sap- and heartwood. Grain fine and even, with well-marked growth rings because of the sharp distinction of narrow layers of latewood. Resin canals absent and individual resin parenchyma cells few.

Stomates in a waxy stomatal band on either side of the midrib on the underside and in individual lines on the upper side. Each stomate sunken beneath the four (to six) surrounding subsidiary cells, which may be shared between adjacent stomates in a single line, without a Florin ring, plugged with wax. Photosynthetic tissue on just one side, the side facing up, corresponding either to the right or left side of ordinary foliage leaves in other conifers depending on whether the leaf is positioned to the left or to the right of the twig. The single resin canal then lying either to the right or left of the midvein rather than below it (although in strict morphological terms it is still on the abaxial side of the midvein, the side away from the twig, just as it is in Podocarpus, for example).

Two species in Fiji and New Caledonia. The species of Acmopyle, although found in a very few botanical gardens, are not in general cultivation, and no cultivar selection has been undertaken.

Acmopyle has a distinctive combination of features, some of which are shared with several other podocarp genera while others are unique to it. The seed cone, with its well-developed podocarpium, is somewhat similar to that of Podocarpus and Dacrycarpus but consists of more bracts. The warty surface of the bracts and their preservation of rudimentary seed scales even in those without seeds is also distinctive. The upright seed leads to a variation on the pollination method found in Podocarpus and many other podocarps in which pollen as scavenged from particular areas of the young seed cone by a very large pollination drop. Rather than having the whole ovule pointed back down toward the cone axis as in most other podocarps, Acmopyle has a special down-curved beak that directs the pollination drop downward onto the correct surfaces, including the uppermost bract, the only one that is completely devoid of any trace of a seed scale.

The type of foliage leaf found on the short shoots, flattened side to side, is very unusual in conifers and, among podocarp genera, is otherwise found only in Falcatifolium and in juvenile foliage of Dacrycarpus. Similar fossilized foliage that can be assigned with come confidence to Acmopyle because it preserves the distinctive structure of the epidermis is known as early as the late Paleocene more than 55 million years ago, making it one of the older living genera of conifers. Much older fossils resembling Acmopyle and dating from the Jurassic, more than 140 million years ago, are not established as belonging to this genus or even as being closely related to it. The verified fossils have been found in Antarctica, Argentina, Tasmania, New South Wales, and Western Australia, all regions outside its present range of distribution but conforming well to a typical southern hemisphere distribution that reflects the late Mesozoic breakup of the southern hemisphere supercontinent Gondwanaland. No fossils of Acmopyle are known from sediments younger than the early Oligocene, more than 30 million years ago, so there is a long, uncharted path from the fossil record to the present-day distribution.

The persistence of the genus may be related, in part, to the presence of nitrogen-fixing nodules on the roots, structures present in some other podocarps as well. Its age might suggest that Acmopyle is not particularly closely related to any other podocarps despite its overlapping features with other genera. Not surprisingly, then, DNA studies do not provide any convincing account of its relationships, but it may be closest to a group consisting of Podocarpus and it near relatives and Dacrydium and its near relatives. This would be in accordance with its resemblance to some of these genera in such things as possession of a podocarpium, while retaining other features that are more primitive within the family, like the greater number of bracts making up the seed cone.





Attribution from: Conifers Garden