Electrophysiology and Phototropism - Communication in Plants: Neuronal Aspects of Plant Life - page 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 veg-
etative and animal kingdoms in protoplasmatic structures earlier than the
morphological differentiation of nervous tissues (Volkov 2000). These pro-
toplasmatic excitable structures consolidated into the organs of a nervous
system and adjusted the interaction of the organism with the environ-
ment.
Volkov and Haack (1995) studied the role of electrical signals induced by
insects in long-distance communication in plants and confirmed the mech-
anism 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 distri-
bution 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 electrochem-
ical signals have reached the cell, deep cytophysiological reactions oc-
cur.
24.2
Phototropism and Photosensors
Lightisanessentialsourceofenergyonwhichmanyofthebiologicalfunc-
tions of plants depend. The sun's radiant energy optimizes germination,
photosynthesis, flowering, and other processes needed to maintain home-
ostasis. The first experiment on phototropism is probably lost in antiquity.
Most likely someone noticed an indoor potted plant bending toward a win-
dow 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 bend-
ing also occurred well below the tip. Darwin (1888) hypothesized that an
“influence” was translocated from the illuminated tip to the part where
bending occurred.