Where Does The Green Go?

 by Kim Woolcock

It’s autumn where I am, and the leaves are turning. They look like they’re setting themselves on fire before they fall, going out in a blaze of glory. Crispy husks carpet the forest floor, ready to be turned into next year’s nutrients. 

 

Leaves of Acer palmatum subsp. matsumurae (Koidz) Ogata

Photo credit: 松岡明芳

I love it, but it also seems extravagant. Why don’t leaves just stay green until they fall?

It turns out the trees are being thrifty. The leaves are full of chloroplasts, which contain lots of chlorophyll, the green light-harvesting pigment that lets plants spin sunlight into sugar. Chlorophyll is expensive, nutrient-wise. It’s loaded with nitrogen, and so trees tuck it away for winter. They break down the chloroplasts, pack the nutrients for transport, and send them to the trunk and roots. When they’re done collecting what they need, they build a waxy wall between the branch and the leaf and then let the leaf drop.

Packing up the green pigment lets the yellow and orange pigments, carotenoids, shine through. They were there all along, helping the leaves capture light of different wavelengths, but they were masked by the green. As the chlorophyll is removed, the carotenoids pick up some of the slack, making as much energy as they can with the last rays of autumn.

Not all leaves turn yellow or orange—some turn flaming red, thanks to anthocyanins. These pigments aren’t there in the summer, but are made specially in the fall. They act as sunscreen, protecting other leaf molecules from sun damage after the chlorophyll’s gone. That’s why they’re brightest in areas where fall days are sunny. They’re also made from leftovers. As the days get shorter, leaves keep producing sugar and sending it to the roots. But when nights get too cold, sugar transport is slowed, and some sugar gets stuck in the leaves, where it’s made into anthocyanins instead.

 

The green is almost gone. Image credit: Sander van der Wel

It’s a big job, getting ready for winter. Consider a single aspen tree (Populus tremula). Researchers made a detailed calendar of fall events for this tree, tracking components such as pigments, metabolites, nutrients, and photosynthesis rate. The tree has several million leaves, each of which contains ~30 million cells. Each cell contains ~40 chloroplasts. So every autumn, the tree has to synchronize the dismantling and transport of 10¹⁵ chloroplasts (one quadrillion, or the total number of ants on earth, just for scale), all in about a month. That’s just one tree. No wonder they look like they’re on fire.

So that’s where the green goes. It’s stashed away in trunks and roots for the winter, waiting to be remobilized in spring. Winter has always seemed drab compared to fall, to me. But knowing this makes me look at winter tree trunks differently – they are actually full of secret green. 

 

Kim Ryall Woolcock is the co-author of Design Like Nature: Biomimicry for a Healthy Planet (Orca, 2021) with Megan Clendenan. Her next book Tough to be Tiny is coming out from Flying Eye Books in July 2022. You can find out more at www.kimwoolcock.com
 

Resources:

John King. 2011. “Reaching for the Sun: How Plants Work, second edition.” Cambridge University Press, Cambridge. 298 pp.

Johanna Keskitalo et al. 2005. “A Cellular Timetable of Autumn Senescence.” Plant Physiology, 139:4, 1635–1648. https://doi.org/10.1104/pp.105.066845

https://www.sciencedaily.com/releases/1997/10/971005050137.htm

 

 

Salty Slug Love

Slippery, slimy, oozy slugs; what’s not to love? Slugs can be small and slugs can be as long as your arm! Slugs can be sausage shaped and brown, or they can have leaves, legs, and lots of rainbow colours! They can live anywhere wet, and lots live under the sea. 

Salty Samples 

Sea slugs are a family of boneless animals that contain a particularly fancy looking molluscs called nudibranchs. Usually they’re small enough to fit on your hand, but they can be as long as a sheet of binder paper. Their shapes and colours result in names like “dragon” and “orange peel,” or “sea bunny,” “dancer” and “clown.” Start an image search and you could browse pictures of fanciful nudibranchs all day. There are more than 3000 kinds! 

Pantry Paint Packs 

Like flamingoes get their pink colour from their food, nudibranchs get their colour from their diet too. Check out this little creature that looks like a sheep that rolled in cut grass! It’s the leafy sheeps’ algae diet that makes them green. They store the chloroplasts from their food and that means photosynthesis happens inside their bodies like it does in plants. 

Nudibranch Brunch 

Nudibranchs are carnivorous! They eat algae, sponges, and even other sea slugs. Some also eat coral and even stinging jellyfish, and that makes them a bit toxic. Like the leafy sheep keeps some chloroplasts from its food, the jellyfish eating “blue dragon” keeps some of the stinging cells from its food. Like a lot of colourful things in nature, the bright hues warn us that they can hurt. Touching them can sting. 

Making Nudibranchs 

Any two nudibranchs can make babies together, because they all have both sex organs. They’re hermaphrodites, just like earthworms and most snails are. About 5 of every 100 animal species are hermaphroditic.

A gooey ribbon of fertilized eggs will hatch into nudibranchs that look just like their parents but smaller. Depending on the type, there can be 2 eggs or 25 million! Once they leave the nest, they’ll live just a few weeks to a year. 

Notice Nudibranchs 

To see a nudibranch in person, you’ll have to go out into the ocean because they don’t survive captivity for long. But you’ll find some of these saltwater slugs along every ocean coast — except in the Arctic and Antarctic circles. They love coral reefs. You will find nudibranchs in shallow water and way down in the deep. Look on the bottom, and remember they’re usually very small. Most photos of these creatures are taken with a close-up macro lens.