References

24 Electrophysiology and Phototropism 353
The huge amount of experimental material testifies that the main laws
of excitability such as the inducement of nonexcitability after excitation
and the summation of subthreshold irritations were developed in the vegetative
and animal kingdoms in protoplasmatic structures earlier than the
morphological differentiation of nervous tissues (Volkov 2000). These protoplasmatic
excitable structures consolidated into the organs of a nervous
system and adjusted the interaction of the organism with the environment.
Volkov and Haack (1995) studied the role of electrical signals induced by
insects in long-distance communication in plants and confirmed the mechanism
by which electrical signals can directly influence both biophysical
and biochemical processes in remote tissues.
Bose (1925) has established the availability of a reflex arch in plants
such as Mimosa pudica. When plants are excited sensory cells generate
impulses which terminate at motor cells. The character of their distribution
depends upon the physiological condition of plants. The signal
from a beam of the sun is transmitted to the tissues of a stem at an
extremely high speed and consequently the stem will curve toward the
source of light. After excitation, the illuminated top of a stem causes an
impulse to be distributed among the tissues (Volkov et al. 2005). When
the impulse reaches motor cells, the stem bends. Thus, after electrochemical
signals have reached the cell, deep cytophysiological reactions occur.
24.2
Phototropism and Photosensors
Lightisanessentialsourceofenergyonwhichmanyofthebiologicalfunctions
of plants depend. The sun’s radiant energy optimizes germination,
photosynthesis, flowering, and other processes needed to maintain homeostasis.
The first experiment on phototropism is probably lost in antiquity.
Most likely someone noticed an indoor potted plant bending toward a window
and rotated the pot 180 ◦ and then noticed later that the plant again
bentbacktowardthewindow.Thefirstscientiststoreallymakeprogress
in explaining phototropism were Charles and Francis Darwin (1888). They
grew canary grass seedling and placed little caps of tinfoil or this glass
painted black on the tips of the coleoptiles and determined what portion
of the coleoptile had to receive light in order for phototropism to occur.
Darwin (1888) found the tip was the light-sensitive part, but the bending
also occurred well below the tip. Darwin (1888) hypothesized that an
“influence” was translocated from the illuminated tip to the part where
bending occurred.