Plants aren't as flashy as animals, but they're complex beings with elaborate survival strategies who actively engage with the environment around them, and sometimes even helping their fellow plants react to danger. Still, it would be rather a stretch to claim that they have "minds." And yet this is what Gustav Fechner believed.
The 19th century German scientist believed that nearly everything had a mind. The highest mind was that of the universe itself, which presided over the lesser minds — those of the stars and planets, of humans, of animals, and finally, of plants. The metaphor is quite pleasing and quaint. A painting of this hierarchy of minds would, depending on the art style, look good on the wall of a nursery, or on the wall of a store in Sedona where people try to sell you crystals. Fechner, however, did not believe in it metaphorically. He believed it in a literal sense, and his belief ended up creating an important and enduring science.
Fechner believed that the mind and body were one. He reasoned that if the mind and body were the same, any physiological change would have to result in a mental one. To find this connection between physical reality and consciousness, Fechner invented the science of psychophysics.
Put a glass of water on a very slightly warm hotplate and the water will slowly warm. A person with one finger in the glass on the hotplate and one finger in a glass that's not being heated will not feel any change at first, but at some point, when the water temperature rises a certain fraction of a degree, they'll notice that one glass is warmer. Take one grain of sand at a time out of a pile held on a person's outstretched hand, and they will at some point register that that pile is lighter than an unreduced pile on their other hand. Fechner was obsessed with the threshold at which the mind perceives the physical world. He measured everything. His typical technique was handing a person two weights, one of which was just slightly lightened over and over, and recording the point at which that person could tell the difference.
It was not a casual interest on Fechner's part. The man made nearly 25,000 measurements during his lifetime.
Fechner investigated other ways the world interacts with the mind. He was fascinated with light and color. Some of his investigations were just like his weight measurements, meant to determine at what point a person could distinguish color or light level differences. He's also discovered some lasting mysteries; for example, he may be the first scientist to have discovered that a moving pattern of black and white dots causes people to "see" colors. (It's now called the Fechner Effect.) He worked with people with synesthesia, trying to determine why they saw colors in music or language.
Meanwhile, Fechner insisted that both he and his students quantify and measure everything they possibly could. He wanted to measure the mind-body connection in order to discover the "mind" in every living and nonliving thing. He never did, but he did open up an entirely new branch of science. That's not bad for a life's work.