The Chinese Arborvitae, Platycladus orientalis,
is a species of Cypress widely grown as an ornamental plant in China, North
Korea and the Russian Far East. Its precise origin is unknown, though it is
thought to be native to Gansu, Hebei, Shaanxi and Shanxi Provinces. Genetic
studies have suggested that the last common ancestor that it shared with its
closest known relative, Microbiota
decussate, lived about 33 million years ago in the Early Oligocene.
In a paper published in the journal PLoS One on 17 December 2014, Jing-Yu
Wu of the Key Laboratory of Mineral Resources in Western China at the School of Earth Sciences and College of Earth and Environmental Sciences at LanzhouUniversity and the State Key Laboratory of Palaeobiology and Stratigraphy at the
Nanjing Institute of Geology and Paleontology, and Su-Ting Ding, Qi-Jia Li, Zhen-Rui
Zhao and Bai-Nian Sun, also of the Key Laboratory of Mineral Resources in
Western China, describe a new species of Platycladus
from the Late Miocene Nanlin Formation in Lianghe County in Yunnan Province.
The new species is named Platycladus yunnanensis,
meaning ‘from Yunnan’. It is described from eight specimens, all of flattened
splays of foliage preserved in mud- and siltstones, from which it was possible
to extract some of the material for examination under a light microscope.
Folliage branch of Platycladus yunnanensis
in matrix. Scale bar is 1 cm. Wu & Ding et
al. (2014).
These are the oldest specimens of Platycladusor Microbiotaknown
(specimens of Microbiota have been
reported from the Pliocene of Russia), and Wu & Ding et al. are able to confidently date the deposits that produced them
to between 6.77 and 7.20 million years old, using potassium/argon isotope
dating of basalt layers above and below the sediments. Argon-Potasium dating relies on determining the ratio of radioactive Potasium⁴⁰ to Argon⁴⁰ within minerals from igneous or
metamorphic rock to determine how long ago
the mineral cooled sufficiently to crystalize. Potasium⁴⁰ is often incorporated into cooling volcanic rocks, whereas any inert Argon present will escape as a gas. No further Potasium⁴⁰ or Argon⁴⁰ will
enter the mineral from this point, but Argon⁴⁰ is produced by the decay
of radioactive Potassium⁴⁰ at a steady rate, enabling scientists to establish a precise date for the crystalization of the minerals containing the two elements.
Foliage branch of the extant Platycladus orientalis. Scale bar is 1 cm. Wu & Ding et al. (2014).
The modern Platycladusorientalisis
thought to be native to areas of China far cooler and drier than Yunnan
Province, though it will live there and has naturalised in parts of the
province and neighbouring Sichaun. Platycladus yunnanensisis
essentially similar to the modern Platycladus orientalis,
though it does have open stomatal pits and shallowly sunken guard cells, which
are generally considered to be adaptations to a warm, moist climate, as would
be predicted for the Late Miocene of Yunnan Province.
(C) Outer view of the stomatal zone in the abaxial
cuticle of facial leafof Platycladus yunnanensis,
showing the Florin rings. (D) Outer view of the stomata in the abaxial cuticle
of lateral leaf, showing the elliptical stomatal pit and surrounding Florin
ring. Wu & Ding et al. (2014).
See also…
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