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Bioelectromagnetism

proposed that these variations might be correlated to plant evolution (Occhipinti et al., 2014). In the

case of plant evolution, the greater incidence of high-energy particles and direct efects of the GMF on

the biological system during the GMF reversal and excursion periods might contribute to alteration

that eventually led to mass extinction (Mafei, 2014). Because plants, in general, do not change their

orientation and habitat once germinated, there might be distinctive action of the terrestrial magnetism

on the growth and physiology of plants (Yamashita et al., 2004). Magnetoreception might be a driving

force contributing to plant evolution, but in order to prove such a hypothesis, further studies should be

needed to confrm that some plant genes are afected by GMF reversals (Mafei, 2014).

By partially simulating GMF reversals with reduced MF, Narayana et al. (2018) suggested that GMF

reduction to near null MF has a signifcant efect on ion content and ion transport gene expression in

Arabidopsis. A few minutes afer exposure to near null MF, plants respond with modulated root content

and gene expression of all nutrient ions under study, indicating the presence of a plant magnetoreceptor

that responds immediately to MF variations by modulating channels, transporters, and genes involved

in mineral nutrition (Narayana et al., 2018). With time, the content of the nutrient ions decreases and

is followed by the typical physiological responses of plants exposed to near null MF, including delay of

fowering time (Xu et al., 2012; Agliassa et al., 2018a), photoreceptor signaling (Xu et al., 2014; Agliassa

et al., 2018b; Vanderstraeten et al., 2018), and seed germination (Soltani et al., 2006). It is interesting to

note that the response to near null MF is very rapid, which suggests that some ion channel and transport

activity might be dependent on magnetoreception systems not necessarily related to gene expression

(Narayana et al., 2018). Ongoing studies are evaluating the role of ferromagnetic, paramagnetic, and

diamagnetic metals on plant magnetoreception (Narayana et al., 2018).

6.2.4 Magnetic Sense of Humans

First of all, historically, how did we get to know the GMF? It was the ancient Chinese people who real­

ized that there was the GMF on the Earth, which exerted a mysterious force to make magnetic materials

point to the “S” pole in the Northern Hemisphere (William, 2007). Te magnetic compass was not, at

frst, used for navigation, but for geomancy and fortune-telling by the Chinese. In China during the Han

Dynasty between the second century BC and frst century AD, primitive compasses were used as des­

ignators of direction that the Chinese primarily used to order and harmonize their environments and

lives (Merrill and McElhinny, 1983). Today we are familiar with this kind of use of direction as part of

“Feng Shui,” an ancient Chinese practice that has evolved into a decorating trend. Although the ancient

Chinese people did not understand why this phenomenon happened, they can be called “the discoverers

of the GMF” in the fact that they used the south pointer compass as a tool to know the direction of the

GMF. Magnetic compasses were later adapted for navigation during the Song Dynasty in the eleventh

century (William, 2007).

An English physician, physicist, and natural philosopher, William Gilbert frst revealed that the main

origin of the GMF is inside the Earth (Gilbert, 1600). He didn’t know the mechanisms, but he was

regarded as the frst discoverer of the GMF in the fact that he noticed that if the Earth were magnets, he

could explain the world distribution of the inclination (dip angle) (Gilbert, 1893). In a bar magnet, the

N and S poles are arranged in a straight line, but even if the magnet is a sphere, an MF similar to that in

the case of a bar magnet appears. By measuring the direction of the MF around a spherical magnet, he

found that the already known distribution of the inclination of the GMF at the time was coincidental

with that of the MF around a spherical magnet.

In addition, a German mathematician and physicist, Carl Friedrich Gauss accurately measured the

GMF strength (Bühler, 1987). He contributed greatly to the elucidation of the mystery of the GMF. He

also knew the method used to analyze gravity in celestial mechanics, so he applied it to the description

of the GMF. In the 1830s, he undertook research on the origin of the GMF. He focused on a “dipole”

in which the GMF becomes weaker at 1/r³ with respect to the distance r. Moreover, he found that the

GMF could be a superposition of infnite felds such as “quadrupole” whose strength becomes weaker