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Cheshunt pine, J. Hooker  1857


Evergreen shrubs or small trees with fibrous bark peeling in strips on multiple main branches from the base. Branchlets alternate, radiating in three dimensions, four-angled. Scaly winter buds absent. Leaves in alternating, overlapping pairs, scalelike, keeled, completely clothing the branchlets, rarely with a resin gland on the outer face. Free tips of leaves shorter than to about as long as the attached leaf bases, tightly pressed against the twig.

Plants dioecious. Pollen cones single at the ends of branchlets, with three or four alternating pairs of pollen scales, each scale with two pollen sacs. Pollen grains small (25-30 µm diameter), spherical, minutely bumpy, sometimes also with a short germinal papilla. Seed cones numerous, single at the tips of ordinary branchlets, maturing in a single season, roughly spherical with two alternating pairs of seed scales and a central column. Cone scales formed by intimate fusion of the bract and seed scale, woody but thin, radiating from the base, and touching but not overlapping when closed. The upper pair of scales each two-seeded, larger than the lower, sterile pair (hence the scientific name, Greek for “two seats”). Seeds oval with a prominent pollination tube at the tip, two equal wings extending the length of and as wide as the body, and a third smaller or rudimentary wing. Wings formed as outgrowths of the seed coat. Cotyledons two, each with one vein. Chromosome base number x = 11.

Wood light, moderately hard, with light-colored sapwood sharply contrasting with purplish brown heartwood. Grain very fine and even, with distinct, very narrow growth rings marked by a fairly abrupt transition to a few rows of latewood tracheids with hardly any central lumen (opening). Resin canals absent but with numerous individual resin parenchyma cells scattered through the growth increment or concentrated into narrow, open bands.

Stomates forming two broad, triangular bands on the hidden, inner (upper) face of the leaves and patches at the base of the outer (lower) face where they are covered by the tips of the previous leaf pair on the twig. Each stomate sunken in a narrow pit between and virtually hidden by the four to six surrounding subsidiary cells, which are topped by a steep, complete Florin ring. Leaf cross section with a single-stranded midvein directly above a single large resin canal (that may not extend far into the free leaf tip) and flanked by small wedges of transfusion tissue. Photosynthetic tissue forming a thin palisade layer beneath the epidermis and accompanying thin hypodermis only in the free tip, the rest made up of spongy mesophyll.

One species in Tasmania. As long believed, Diselma has been shown using DNA sequences to be closely related to South American Fitzroya but is closest to African Widdringtonia. It is somewhat like a miniaturized version of Widdringtonia, in much the same way as Microbiota resembles a smaller version of Platycladus. Diselma archeri is too small to yield lumber and has no other economic uses, despite its ecological importance in subalpine Tasmania, it is not in general cultivation and there has been no cultivar selection. Unlike many southern hemisphere members of the Cupressaceae, there is no known fossil record of Diselma, so the taxonomic and geographic history of its divergence from a common ancestor with Fitzroya and Widdringtonia is entirely speculative.




Attribution from: Conifers Garden