Heliotrope is named for its behavior: it turns toward the sun (helios in Greek) as it crosses the sky. The trope part of the name comes from the Greek root tropos, meaning turn. A tropism is an instinctive response to some stimulus, often a turning toward or away from something. Heliotrope has also gone by the name turnsole, which is related to the Latin word for the sun, sol. Because some types of daisy open and close with the rising and setting of the sun, its name comes from the Old English for day's eye.
|Daisy, or day's eye; some types open in the sunshine and close in the dark.|
Tropism, by the way, is one of those fun words that you can make new words from by adding an appropriate prefix. Scientists talk about thermotropism (attraction to heat) and phototropism (attraction to light). I talk jokingly about bibliotropism, my own tendency to drift toward the books in any room.
I also learned recently that the word tropic comes from the same Greek root, tropos. I'm reading the sixth book in Colleen McCullough's Masters of Rome series (highly recommended, by the way), and I got to the part where Caesar and Cleopatra were traveling up the Nile together. They were having a well-informed discussion of the natural history of the region, as political leaders are apt to do when on a junket (aren't they?). Caesar mentioned that Eratosthenes had given the Tropic of Cancer its name because it marked the point where the sun reached its northernmost position, on the summer solstice, before it turned around and headed south again in its annual cycle. (Of course the sun doesn't really turn around, but that's how it looks.)1
The moon and stars are also well-represented among the flowers. The moonflowers are a confusing tribe, including various unrelated plants that bloom at night. For example, the genus Hylocereus is a type of cactus that produces big gorgeous flowers, often only one flower on one night a year. It's also called the night-blooming cereus (although other things are also called that). If you've heard of someone waiting eagerly for a night-blooming cereus to finally blossom, it was probably a Hylocereus undatus.
Another moonflower, the genus Datura, produces lovely nocturnal blooms and potentially lethal alkaloids; it's also known as the angel's trumpet for its trumpet-shaped flowers. Datura is a member of the Solanaceae family, AKA the nightshades. We'll be meeting this large and varied family again; it contains a wide range of plants, from the potatoes and tomatoes and eggplant on the table to the petunias in the garden and the tobacco, datura, and deadly nightshade that entice and kill. Interestingly, the name Solanaceae might also come from the Latin sol (I've seen several possible reasons for this). There are other explanations, but if it's true, it's curious that a family named for the sun should also be called nightshades.
The Mentzelia are another type of moonflower, although they're also called evening stars or, most commonly, blazing stars, because they not only bloom at night but have a star-like shape. They're also called stickleaf; the leaves contain many small barbs that enable them to stick to fur, socks, pants, etc., although it's not clear what use this might be to the plant.
With the Mentzelia, we can turn again to flowers that are named not for when they bloom but for their appearance. There is an entire family named for its star-shaped flowers, the Asteraceae. The Mentzelia are not members of it; however, the Asteraceae do include another blazing star, liatris. Along with the asters, those starry flowers of late summer and fall, the family also includes dandelions, artemisia, zinnias, marigolds, and black-eyed Susans. They also take us back to where we started, because sunflowers and most daisies are members of the Asteraceae family.
Thus ends our whirlwind tour of heavenly bodies on Earth. If you have a favorite that I haven't covered, please post it in the comments.
1 The Tropic of Cancer, at about 23.5° N, is the northernmost latitude at which you can see the sun directly overhead when it's at the zenith (roughly noon); it happens only once a year, on the northern summer solstice. When the Tropic of Cancer was named, the sun was entering the constellation of Cancer on the northern summer solstice; because of precession (the slow wobble in Earth's rotational axis), the sun is now in Taurus on that date.↩