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 What's that word again? The soft rustling of wind in the leaves that lulls you on a summer day. . . .

Sometimes you need just the right word to describe a natural phenomenon. I'm talking a $64,000 word. A word that cleaves right to the heart of the matter and unfurls in your brain with beautiful precision. Here are three unusual plant-related words that you will enjoy if, like me, you have a penchant for collecting words as well as plants.

Psithurism

Psithurism (pronounced SITH-ur-izm) is a word I only recently came across, and fell in love with immediately. From the Greek word for whispering, it is an onomatopoetic descriptor of the sound of wind in the trees and the rustling of leaves. It is the sound of a summer afternoon's nap in a hammock, or an open window on a breezy evening as a storm blows in. I like it for its specificity. Sure, you can use susurrus to describe a whispering or rustling sound, but once you know about psithurism, the particular source of the noise becomes wind in leaves. It's a sound we all know; now we know the perfect word for it.

Marcescence

If you live in an area where American beech (Fagus grandifolia) or oaks (Quercus spp.) grow, here is another phenomenon with which you may be familiar.  Marcescence (mar-SESS-uhnts) is a tree's habit of holding onto withered leaves throughout the winter that it would normally shed during autumn leaf drop.  Both marcescence and its cousin senescence, the process by which cells age and stop reproducing, come from Latin words meaning to wither (marcescere) and to grow old (senescere).  

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Marcescent beech trees in winter hold onto their leaves, unlike most deciduous trees.

Beeches, oaks, hornbeams (Carpinus), and witch-hazels (Hamamelis) share marcescent traits. Some trees will lose the trait as they age, so if you look in a beech forest, you may only see saplings exhibiting this behavior, while mature trees drop their leaves. Some trees retain their leaves only on lower, inner branches in what is known as the "cone of juvenility." Another great phrase!

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 Cone of juvenility. Photo credit: Josh Coceano, Mani Verde

The cone of juvenility is ontogenetically the youngest part of the tree, even though the most recent growth occurs above and beyond it. Yes, that's right, another fantastic word thrown in for good measure: ontogenesis. This term refers to the development of an organism from the earliest stage to maturity. Imagine a time-lapse photo of the tree above, at 10 years, 20 years, and 30 years. Now consider the cone of juvenility as the area of the tree that has existed since perhaps the 10-year mark.  Even though the topmost/outermost twigs emerged at 30 years, the "youngest" part of the tree as it developed is shown by the marcescent area closer to the bottom.

Scientists do not know for sure why some trees possess marcescent traits. There are several interesting hypotheses. One posits that young trees hold onto their withered leaves to deter grazing animals from eating the buds and twigs. Another holds that marcescent trees tend to dominate in drier conditions, and that they create an advantage by waiting to drop their leaves until spring, the perfect time for fallen leaves to decompose and act as mulch. Related to this, a third theory is that marcescent trees tend to have tough, leathery leaves, and the additional weathering from sunlight, wind, and moisture prior to leaf fall allows them to decompose faster once they have dropped to the ground in spring. Finally, some scientists believe that marcescent leaves could provide the trees protection against water or temperature stress. 

Whatever the reason for it, marcescence sure provides more opportunity for psithurism throughout the winter months.

Thigmotropism

Some could argue that this is just a technical term, but it's still unusual in everyday speech. Thigmotropism refers to a plant's directional growth in response to a touch stimulus; in other words, the expression of a plant's sense of touch. Here is a video that nicely captures the phenomenon of thigmotropism:

As the vine grows, it waves around (in a motion known as circumnutation . . . I know!!) until it touches a solid object. In reaction, it begins to grow in a spiraling fashion. Physiologically, the side of the shoot in contact with a solid object grows more slowly than the other side, and as it grows upward, it will form a spiral.  

Both shoots and roots demonstrate thigmotropic responses; this allows roots to penetrate soil quickly instead of getting blocked by stones or other obstacles. Usually, roots have a negative thigmotropic tendency: that is, when they come into contact with a solid object, they grow away from it.

Don't forget . . . 

I've been telling everyone I know about psithurism so that I don't forget the word. Now I've shared it with you, too. If we use unusual words in conversation, we will learn them better. Don't let lethologica overtake you. That's the inability to remember a particular word when it's on the tip of your tongue. Whoa. An unusual word for forgetting unusual words. What could be more aggravating better?

Do you have other unusual plant-related words to share? Pass them along in the comments section. Thanks!