Plasma

3753

Platinum

the vital processes in winter, which allows
of the accumulation of reserve material

Respiration, or the absorption of oxygen,
and the evolution of carbon dioxide, is as
necessary for plants as for animals. When a
plant is deprived of oxygen, all vital proc-
esses are suspended; and if it is kept in the
same atmosphere, the destruction and disor-
ganization of the living substance inevitably
follow.

Nutrition is a general term, covering all
the processes by which the plant body is
built up, from the absorption of nutritive
material in solution from the soil to the ac-
cumulation of reserve. Carbon is derived by
a plant from the atmosphere, chiefly by the
leaves; oxygen and hydrogen are obtained
from the water absorbed by the roots, which
abo brings in the necessary mineral sub-
stances from the soil.

Reproduction, which implies separation,
rejuvenation, and multiplication of the indi-
vidual, is effected asexually and sexually. Asex-
ual reproduction may take place by the divi-
sion of a single cell, when growth limits are
reached, as in the single-celled algse; by the
formation of spores; or by vegetative increase
by stolons, rhizomes, or tubers; and to this
third method the general rame of budding is
applied. Sexual reproduction is more com-
plicated; but, reduced to its simplest terms,
ft consists in the development of two sexual
cells or garnets, neither of which can of itself
give rise to a new organism. But from the
fusion of these two cells a third cell (the
zygote) is produced, which is the starting-
ooint of a new plant. The process is some-
what masked by the alternation of genera-
tions. See BOTANY, FUNGI, FLOWERS, FRUITS.

Plasma. See Blood.

Plasma, a variety of chalcedony, which has
a dark-green or leek-green color, due to the
admixture of minerals belonging usually to
the chlorite group. It takes a good polish. At
present it is principally obtained from India,
where it occurs in the cavities of weathered
igneous rocks.

Plaster and Plastering-, The application
of a coat of plastic material to the surfaces of
masonry or of woodwork, for the purpose of
bringing the latter to a sufficiently smooth
surface to receive surface decoration, is
known as plastering. Upon ceilings, or wood-
en partitions, the several coats are applied
upon laths of wood or on wire netting or
lathing. In ceilings, however, it is customary
to add a finishing coat., containing calcium

sulphate, or of what is commonly called plas-
ter of Paris.

Plaster of Paris, obtained by cautiously
heating gypsum in kilns or continuous fur-
naces out of contact with fuel to about 120°
c It is a fine white powder that sets rapidly
with expansion to a hard solid after being
mixed into a paste with water. It is utilized
for cementing objects together, and for copy-
ing objects of every description.

Plastics, a large group of organic, often
synthetic, materials. Some are proteins, as
nylon; some are cellulose derivatives, as Plas-
tacele; and some are resins formed by poly-
merization, as Lucite and Plexiglas. They are
cast or molded and used for making many
articles. Celluloid and Bakelitc are early plas-
tics. World War II spurred research on old
and search for new plastics.

PJata, Rio de la, or River Plate, inlet of
e. of S. America, between Argentina and
Uruguay, forming the estuary of the Parana
and Uruguay rivers. Length, 130 miles.

Platsea, a city in Boeotia, ancient Greece, at
the northern base of Mount Cithaeron. Its
history turns on its refusal to join the league
of Boeotian cities dominated by Thebes. In
510 or 509 B.C. it formed an alliance with
Athens. The Athenians had at once to fight
the Boeotians. The Peloponnesian War began
with an unsuccessful attempt (431 B.C.) by
the Thebans to seize Platsea. The Pelopon-
nesians besieged Platsea (429-27 B.C.) and
captured it by starvation, the town being
razed to the ground.

Platamts, a genus of trees belonging to the
order Plantanaceas. The sycamore or button-
wood (P. occidentalis) has coriaceous, pu-
bescent leaves. The bark of this sycamore has
the habit of splitting off in thin, broad scales,
leaving the upper part of trunk and branches
blotched with white. Its round balls of fruit
hang on the trees over the winter. It is found
in rich soil, particularly in moist lands along
streams, and reaches to a great height (130
ft.) and girth (50 ft.); its reddish-brown
wood is used chiefly in making cigar-boxes,
although compact, hard, and difficult to work,

Plateau means either a high level plain, or
an elevated part of a mountain system shut
in by bordering chains, and in some cases
traversed by mountain ranges or by a table-
land. A submarine plateau is a steeply bor-
dered elevation in the sea floor.

Plating. See Electro-deposition.

Platinum (Pt, 195.2), a metallic element
occurring in alluvial deposits or in rock-form-