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Pherosphaera, W. Archer bis 1850

Pherosphaera description


Evergreen shrubs of varying habit and crown form, without a single main trunk. Bark obscurely fibrous, scaly, finally becoming somewhat furrowed. –branchlets all elongate, without distinction into long and short shoots, hairless, grooved between the attached leaf bases and remaining green for the first year but generally hidden by the foliage. Resting buds indistinct, consisting solely of as yet unexpanded ordinary foliage leaves. Leaves spirally attached. Juvenile and adult foliage similar. Leaves of both types scalelike or needlelike, a little flattened top to bottom, rounded to roundly angled on the outer face, standing out from or pressed against the twigs, with a very narrow, thin, papery margin.

Plants dioecious. Pollen cones single at the tips of otherwise ordinary foliage shoots. Each pollen cone spherical to egg-shaped, without distinctive bracts at the base and with about 8-25 spirally arranged, rounded pollen scales, each scale with two pollen sacs. Pollen grains small (body 25-40 µm long, 30-45 µm overall), with three round, smooth and inconspicuously internally sculptured air bladders, which extend a little beyond and are tucked under and touch each other beneath the conspicuously larger, essentially the tips of very short, initially conspicuously down-curved shoots bearing ordinary foliage leaves, which later straighten and right the cones. Each seed cone somewhat reduced and modified but still conelike, with about 6-16 fairly tightly spirally arranged bracts that do not swell and unite to form a podocarpium. Middle two to eight bracts fertile, each bearing one upright seed that is apparently inserted directly on the bract without any obvious seed scale (epimatium). Seed hard, a little flattened, with three ribs, ending in a broad, cup-shaped, straight beak, maturing in a single season. Cotyledons two, each with two veins. Chromosome base number x = 13.

Wood dense, the narrow, light brown sapwood sharply contrasting with the darker heartwood. Grain very fine and even, with evident growth rings marked by a fairly abrupt transition to a narrow band of darker latewood. Resin canals absent but with a few individual resin cells.

Inner (upper) face of leaves with many lines of stomates making up a single, broad stomatal band. Each stomate sunken beneath and largely hidden by the four (to six) subsidiary cells, which are topped by a prominently raised, continuous or interrupted Florin ring and plugged with wax. Midvein single, inconspicuously buried in the leaf tissue of the scale leaves or raised as a midrib in the needlelike leaves, with one resin canal immediately beneath it and small wedges of transfusion tissue at the sides. Photosynthetic tissue with a palisade layer on the outer (lower) face of the leaf beneath the epidermis and accompanying hypodermis, the remaining tissue consisting of spongy mesophyll reaching the stomates.

Two species in southeastern Australia. The three scale-leaved, shrubby subalpine conifer species of Tasmania, belonging to three different genera in two families (Podocarpaceae and Cupressaceae), are so similar in general appearance that it is hardly surprising that they were confused with one another when they were first described botanically in the mid- 19th century based on just a handful of specimens. For a century more, the two species of Pherosphaera were mistakenly referred to as Microstrobos, mistakenly because the type specimens of the latter name are actually samples of strawberry pine (Microcachrys tetragona), another of the confusing Tasmanian species. Thus, although the name Microstrobos was universally used for the plants now referred to Pherosphaera and was never applied to strawberry pine, it is formally a synonym of Microcachrys rather than of Pherosphaera. This interpretation of the names, first advanced in 1951, was challenged in 2005 by a careful but not unequivocal analysis of wording in the original literature. Whether we return to use of the name Microstrobos for this genus must await decisions by the nomenclatural section of an International Botanical Congress. Like the closely related Microcachrys, Pherosphaera bears fairly conelike seed cones, unlike the highly modified and reduced, variously fleshy, bird-dispersed ones characteristic of most podocarps. Nonetheless, the seed cones are rather small compared to those of many conifers in other families, especially Araucariaceae in the southern hemisphere or Pinaceae in the northern (hence the scientific name, Greek for “small cone”). The two extant species of Pherosphaera are sparingly cultivated, primarily in botanical gardens, and there has been no cultivar selection.

Since pollen grains of Pherosphaera can be difficult to distinguish from those of Microcachrys, the fossil pollen record of the genus is somewhat ambiguous. Fossil twigs of the two genera, in contrast, are readily distinguishable, and a somewhat meager fossil record of shoots of Pherosphaera has been discovered in Tasmanian sediments dating back to the Eocene, some 50 million years ago. Some of these fossils occur with assemblages of plants that imply much warmer conditions than any now occupied by the extant Mount Mawson pine (Pherosphaera hookeriana).




Attribution from: Conifers Garden