The professor is talking about plant hormones. The word hormone is derived from a greek verb. That means to excite hormones are found in all multicellular organisms and function to coordinate the parts of the organism. A hormone is a chemical signal. It's produced by one part of the body, and is then transported to other parts of the body, where it triggers responses in cells and tissues. The concept of chemical messengers in plants first emerged from a series of classic experiments on how plant stems respond to light. Think about this. A house plant on a window sill grows toward light. If you rotate the plant, it will soon reorient its growth until its leaves again face the window. The growth of a plant toward light is called photo tropism. In a forest or other natural ecosystem where plants may be crowded. Photo tropism directs growing seedlings toward the sunlight that powers photosynthesis. Some of the earliest experiments on photo tropism were conducted in the late nineteenth century by Charles darwin and his son, francis, the darwin zablocki va, that a grass seedling could bend toward light only if the tip of the shoot was present. If the tip was removed, the shoot would not curve toward light. The seedling would also fail to grow toward light. If the tip was covered with an opaque cap, the darwin spur post the hypothesis that some signal was transmitted downward from the tip into the part of the stem that controlled growth. Later experiments by other scientists studying phototropism led to the discovery of chemical messengers that stimulated growth in the stem. These chemical messengers were hormones.