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• $40.50 for a lawn sprinkler startup for up to 8 zones ($80 value)
$78.50 for a lawn sprinkler startup for up to 16 zones ($175 value)

Plant Growth: Moving Without Muscles

Silently and stealthily, plants are plotting to take over your yard. Explore the forces that set greenery in motion with Groupon's study of plant growth.

Phototropism: How a plant grows in response to light. Most plants naturally grow toward a light source, soaking up its rays to produce energy. If a houseplant is placed on the dark side of a windowsill, over time, it will creep as close as possible to the nearest light source, even if it has to bend at unnatural angles to do so. A hormone called auxin triggers this bend—it tends to pool in the shadiest part of the stem, where it spurs new cells to grow quickly and in elongated shapes so that they can reach the light more quickly.

Heliotropism: How a plant stalks the sun. Like phototropism, heliotropism is light-triggered and uses auxin, but it involves no growth whatsoever. Instead, heliotropic plants such as snow buttercups rotate their flowers to physically track the sun across the sky each day. This helps them maximize their UV-feasting time on short winter days, and turns them into a sort of sauna that attracts pollinating insects on vacation from Sweden. Contrary to popular belief, sunflowers are not heliotropic. Before they flower, their buds do track the sun, but it is classic phototropism that leads them to face ever eastward once they have bloomed.

Gravitropism: How a plant avoids growing upside down. A plant can tell when it’s working against gravity thanks to starch grains stored within special cells. If a plant is placed on its side, these grains will fall in the direction of gravity, signaling auxin to report for duty. A rush of auxin will speed up cell growth in key areas until the roots again bend downward and the stem bends upward. In space, plants have been known to send their roots out every which way, though they eventually can compensate for the lack of gravity by relying more on phototropism and hydrotropism (reaching their roots toward a water source).

Thigmotropism: How a plant responds to touch. A powerful sense of touch is what lets vines and ivy climb lampposts and lattices. Once their tendrils brush up against a suitable object, they immediately release auxin to help them curl around it in a tight embrace. Often, plant roots will use thigmotropism to move away from barriers to growth such as house foundations and "No Plants Allowed" signs.