Plants search for food as if they had eyes. ~ German chemist Justus von Liebig
Plants can tell the level of light, its specific direction(s), and its wavelength composition. From this, plants know the time of day, and the time of year.
Photosynthesis produces sugar. One way that plants know the time of day is by the rate of sugar production, which they continuously tally.
Angiosperms use the sense of season to time their flowering. To optimize the prospect for propagation, plants synchronize flowering with pollinator life cycles.
All photosynthetic plants grow toward light, thereby enhancing the photonic fuel for photosynthesis. This is phototropism. Plants adjust growth direction by elongating the cells of the stem on the side farthest from the light source.
A plant knows whether another plant has grown over it. Its typical response is to grow faster, to regain access to better light.
Besides being an energy source, light serves as an important signal to rationally regulate growth. A wide variety of processes throughout a plant are mediated by light-signaling molecules. Even roots are affected.
Light is first detected by photoreceptors in the shoots of a plant. Roots have low-wavelength photonic receptors that are activated by light transmitted from the shoots via vascular bundles. Thus, the entire plant is exposed to light cycles and can plan accordingly.
In essence, plants see, though their sight is naturally oriented toward what a plant needs to know. But then, animal vision is the same in its orientation: limited to spectral bands with needed information, while not able to sense frequencies that are superfluous to survival.
Facing the Dawn
From dawn to dusk, many flowering plants track the Sun across the sky. Plants with leaf mosaics, such as trees and vines, carefully contrive to arrange their leaves so that each gets its fair share of sunlight.
The mallow plant keeps its leaves flush to the Sun all day long, soaking up the light. After the Sun sets, a mallow spreads it leaves conventionally: facing upwards. But as dawn comes due, a mallow turns its leaves to the east in anticipation of sunrise. Such behavior is typical of Sun followers.
Many flowering plants are quite sensitive to light. While plants generally like light, there can be too much of a good thing.
The scarlet pimpernel opens its flowers at dawn, closing them after lunch time. In contrast, the evening primrose keeps its flowers closed during daylight, instead opening as dusk draws on.
Several plants vary their sunlight exposure based upon their ability to take the heat. The sirato orients its leaves to fully face the Sun when moisture is abundant. During drought, a sirato holds it leaves edge-on to the light, to minimize evaporation.
The compass plant, which grows on the North American prairies, takes a simple approach to getting just enough Sun. As flat new leaves grow, they align themselves on a north-south axis. All the leaves of the compass plant lie parallel to each other.
The Sun rises shining directly on one side of a leaf. At noon, light hits the leaves edge-on. As the day wears into the afternoon, the Sun moves around to shine on the other side of the leaf.
Compass plant leaves do not move. Their careful orientation means that they sunbathe in the morning and afternoon, while avoiding the scorching noonday Sun.