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Juniper, Linnaeus 1753


Evergreen trees or tall to dwarf shrubs with a single main trunk or multistemmed from the base. Bark fibrous, usually furrowed and peeling in vertical strips but sometimes flaking and forming rectangular blocks. Shoot system weakly differentiated into main axes and side shoots (branchlets), usually branching in three dimensions. Leaves in alternating pairs or trios, of two main types. All juvenile junipers and adults of all species of section Juniperus and of a few species of section Sabina bearing flattened, linear, needlelike leaves with (one or) two prominent stomatal bands above. Needle leaves of section Juniperus jointed to the twig, while those of section Sabina run down onto it without a joint. Scalelike leaves of most adults of section Sabina closely enclosing the twig, the tips either pressed against it or pointing outward, sometimes with a prominent round or elongate resin gland in the middle or near the tip or base of the outer face. Successive pairs or trios of leaves on main shoots (whip leaves) well separated from one another with the conspicuous, long leaf bases clothing the stem. Leaves on branchlets crowded, their tips hiding the short bases of the next pair or trio along the twig.

Plants usually dioecious, but some species monoecious or with various proportions of plants with either pollen or seed cones or both together. Pollen and seed cones (pollen cones clustered in Juniperus drupacea), attached directly in the axils of needlelike leaves in section Juniperus and at the ends of otherwise ordinary branchlets in section Sabina. Pollen cones spherical to oblong, with three to seven alternating pairs or trios of pollen scales. Each scale with two to eight (often four) pollen sacs on the inner side of the base of the roughly heart-shaped blade at the tip a thin stalk. Pollen grains small (20-40 µm in diameter), spherical, sometimes with an obscure, smooth germination pore, otherwise almost featureless or with minute bumps. Seed cones spherical or approximately so, berrylike (a galbulus), composed of one to three alternating pairs or trios of fused seed scales of varying textures from fleshy to woody, from pink or red to blue or black at maturity, often with a thin waxy coating, maturing in one or two seasons and not opening at maturity. Seeds up to three per scale but only one per cone in many species, ovoid to angular, without wings, often with a prominent attachment scar. Cotyledons two, four, or six, each with one vein. Chromosome base number x = 11.

Wood fragrant, variously soft to moderately hard and medium to heavy, with white to light brown or yellowish brown sapwood sharply contrasting with the rich brown, reddish brown, or purplish brown heartwood, often streaked with brighter or darker hues and with an irregular boundary. Grain typically very fine to fairly fine and very even to somewhat waxy and irregular, sometimes with weak or false rings but usually with well-defined growth rings marked by an abrupt (or sometimes gradual) transition to much smaller but often not much thicker walled latewood tracheids. Resin canals absent but with abundant to sparse individual resin parenchyma cells scattered through the growth increment or more often concentrated in narrow open bands near their middle or in the outer half.

Stomates of scale leaves forming a large patch on each side of the midrib on the upper (inner) face and smaller patches on the hidden portions of the lower face. Each stomate sunken beneath and largely hidden by the four to six surrounding subsidiary cells, which are topped by a continuous but uneven, low Florin ring that may be surrounded by a shallow moatlike furrow. Leaf dross section with a single-stranded midvein above a single large resin canal (that may scarcely extend into the free leaf tip) and flanked by wedges of transfusion tissue. Photosynthetic tissue forming a thin, ill-defined palisade layer beneath the epidermis and adjacent single-layered, continuous (except beneath the stomates), hypodermis on the lower (outer) leaf face (or sometimes on both faces). Palisade layer much looser than usual and giving way smoothly to the spongy mesophyll that fills the bulk of the leaf volume.

Sixty-three species throughout most of the northern hemisphere and in East Africa. Juniperus has by far the most species and the broadest ecological and geographic range of any genus in the Cupressaceae. The relative drought tolerance and fleshy, bird-dispersed cones are often cited as factor responsible for this diversity, but the related genera Cupressus and Callitris are equally drought tolerant, and Taxus is similarly bird-dispersed, yet each of these has far fewer common name for a species in the genus, probably either common juniper (Juniperus communis) or prickly juniper (Juniperus oxycedrus), both of which may be found around Rome.

The natural variability of the genus is augmented in cultivation, where about a dozen species gave rise to most of the numerous cultivars available. Cultivar selection has been very wide ranging, with variations on growth habit from the narrowest, pencil-like spires to cascading creepers closely hugging the ground, including upright weepers, teardrops, and broadly cylindrical spreaders, of every size from tight, dwarf buns to significant (if rarely stately) trees. There are even contortionists, like the well-known Hollywood juniper (Juniperus chinensis ‘Kaizuka’), so prominent in the urban landscape of California. Foliage variants include juvenile and changeable forms (in section Sabina), variations in needle length (section Juniperus), and a wide spectrum of colors from pale to dark greens and light grayish greens to intense blues on the one hand or yellowish greens to deep golden sunshines on the other, not to mention variegation or transient blushes of white to silver or gold. No genus of conifers provides more variety of landscape forms and uses in its cultivars than Juniperus.

Some of the cultivars are selections from natural hybrid swarms, and natural hybridization between species is common in the genus in places where the ranges of related species overlap. Variability, hybridization, and inconvenient political boundaries all led to continuing taxonomic confusion in Juniperus, although analyses of DNA and volatile oils help clarify many difficulties.

DNA studies also show that the genus arose from a Cupressus-like ancestor and that it may be more closely related to either the New World cypresses or to the Old World ones (the results are ambiguous) than these two groups of cypresses are to each other, the latter result being perhaps the most unexpected one found to date by molecular studies of conifers.

Fragments of juniper twigs and seed cones found in ice-age pack-rat middens in southwestern North America are very helpful in reconstructing changes in plant communities and vegetation during glacial and interglacial cycles. Older fossils, however, have been elusive. Most fossils reported as Tertiary records of Juniperus subsequently prove to belong to other conifers, like Glyptostrobus. The wide distribution and large number of species in the genus could thus be more recent geologically than the diversification of many other conifers. A record from the late Oligocene or early Miocene, about 23 million years ago, is the oldest reasonably secure fossil, considerably later than a much less convincing Paleocene record more than 55 million years old.




Attribution from: Conifers Garden