PHYSICS

force is called gravitropism. The roots which grow toward the
center of the earth, demonstrate positive gravitropism; while
shoots, which grow away from the center of the earth, exhibit
negative gravitropism. When the plant is set on a rapidly rotating
turn-table, the roots grow away from the center just as they normally
grow towards the center of the earth (fig. 5.8). Specific
characteristics of gravitropism are: the force of gravity is constant;
there are no gradients of gravity; and it is not possible to switch
on or off the force of gravity.
I
wi
I
orr
~
g R
Fig. 5.8. The plant demonstrates
the growth at an angle eto the
vertical if a flower-pot is set on
a rapidly rotating turn-table;
this angle 8 is determined by
the resultant R of the
gravitational g and inertial orr
accelerations
5.2.2. Effect of Mechanical Factors on Plants
Many plants exhibit nastic movements, which occur in response
to external stimuli such as touch, vibration, mechanical
injuries, light, and chemical treatment. The direction of nastic
movements does not depend on the direction of stimulus. It is necessary
to distinguish the very rapid seismonastic movements and
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Gravitational Orientation in
Microorganisms. Light and gravity
are the most important
external factors for motile microorganisms
which are faced
with a problem of selecting the
best conditions for their survival.
In addition to photomovement,
which depends on parameters
of light, such as direction,
intensity, gradients of intensity
and wavelength, many
cells show gravitaxis - the ability
of microorganisms to orient
their direction of movement
relatively to the gravitational
field of the earth. Algae, such as
Euglena gracilis, Chlamydomonas
nivalis, Cryptomonas, Peridinium
gatunense, P. faeroense, Amphidinium caterea, Prorocentrum micans,
Dunalialla salina, demonstrate gravitaxis, allowing them to
establish vertical distribution patterns. They are capable of active
movement and daily vertical migration of up to 15 m.
the thigmonastic movements. The first type of movement is caused
by touch. For example, when a plant (Mimosa pudica) is touched,
its leaflets rapidly fold downwards in just seconds. This type of
response is used as a way of protection against the wind and
insects and other herbivores. Seismonastic movements are related
to reversible turgor changes in the pulvini - specialized cells at
the base of the leaflets.
Thigmonastic movements are related to a response of some
plants to mechanical stimulation such as shaking, falling raindrops,
mechanical wounding, cutting. For example, Pisum sativum
and Passiflora coerulea, display this type of response to
mechanical stimuli.
5.2.3. Plant Response to Wind
In addition to the stimulation of nastic movements in plants,
the wind is involved in heat and mass transfer, changes the
boundary layer resistance, and the rate of evaporation. Wind can
also induce significant asymmetry in plant architecture by way of
either direct damage (breaking of stems or foliage) or indirect
damage via materials such as salt and sand transported by it. A very
important agricultural problem is lodging of cereals which can
cause a decrease in harvestable yield due to poor light penetration
to the canopy, damaged conducting system, and weakening of
photosynthetic activity of the plant. Also noteworthy is that wind
mediates the deposition and dispersion of soil, plant pollen,
seeds, spores, and droplets of agrochemical substances.
VOCABULARY
English Ukrainian English Ukrainian
Allometry Anouerpis Moment MOMeHT
of inertia inepnii
Angular Kyrose Muscle M'513
displacement 3MIllleHH5I
Average Cepenac Muscle spin M'5130Be
acceleration rrpI1CKOpeHH5I sepereno
Average angular Cepenns xyroaa Myofibril Mio¢i6pina
velocitv
umazrxicrt,
Average velocity Cepezms Neuromast Heapoxracr
lllBI1,nKICTb
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