Glossary

The Science behind the Beauty of it all

Autumn, and its colorful cascade of leaves, are no accident!

Out here in West Texas most of the leaves just turn yellow and drop when we get our first cold snap. However, there are some plants that do put forth some beauty and here is what we have in our back yard. Called a Virginia creeper (how it made it all the way to West Texas is a lot of creeping along.)

It has also grown up into the big tree on our neighbor’s side of the fence that has been there for 50+ years. So it provides a bright color among its leaves also. I remember the big mulberry trees we had in our yard in west Phoenix and the tons of leaves that turned yellow and then fell. Fun when we were young because we could play in them, pain in the arse when we grew up and had to rake them and keep them out of the pool during the winter.

But did you know in Autumn trees don’t lose their leaves—they loose them.

From the following article: http://creation.com/autumn-leaves-fall-by-design. They describe the process far better than I can. It’s basic biology for a high school student (unless their school follows the CCC (Common Core Curriculum.)

The place where the leaf separates (abscises) from the tree is typically located at the base of the leaf stalk (petiole). It is called the abscission zone (AZ). The AZ is no random fracture point but is actually built-in, “pre-positioned”[1] during leaf formation.

With the post-summer hint of coolness in the air, before the onset of wintry weather, trees initiate a “senescence sequence” to systematically retrieve the re-usable resources from the leaves. As this process begins, and the green chlorophyll pigment and other parts of the light-harvesting (photosynthetic) complex[2] are dismantled, the leaf changes color.

First, the formerly hidden carotenoid pigments (e.g. yellow xanthophylls and orange beta-carotene) are now revealed, turning the leaves an orange-yellow hue, as the normally-dominant green chlorophyll fades.

Then, when about half the chlorophyll has been degraded, and as the level of phosphate in the leaf drops, the production of anthocyanin pigments increases. Anthocyanins tint autumn leaves red-purple, and blend with the carotenoids to create the breathtakingly beautiful deeper orange and fiery red coloration

The valuable materials that the tree extracts from the leaves before leaf drop are stored during winter in the tree’s roots, trunk, and branches until next spring when they are ‘recycled’ to re-leaf the tree. This has to be done during the milder weather of autumn, because there can be no retrieval from leaves after the first hard freeze arrives.

With the pulling back of resources from the leaf now completed, the abscission zone becomes a hotbed of activity, in three locations:
1) a cell wall degradation area;
2) a shear force generation area; and
3) a tree protection zone.

All of these must be in place for successful leaf shedding and effective tree survival.

With the abscission process triggered by a raft of chemical signals (including, it is believed, ethylene produced by the internally-gutted leaf), AZ cells start to secrete enzymes.[3] These dissolve the ‘glue’[4] that holds cells together and degrade the primary wall between cells. The surrounding AZ cells actively produce the necessary abscission materials throughout; i.e., they remain alive and active until abscission is completed.

While the AZ can be 5–40 cells wide, within that zone only a band 1–3 cells wide will disconnect from each other to form the fracture line. The weakening of the walls of those cells, coupled with increasing internal water pressure inside the cells, causes the cells to swell. This expansion generates tremendous shear forces, i.e., pushing and pulling on surrounding weakened cell walls, mechanically opening up fracture lines between cell walls. Wind tugging on the leaves helps these fracture lines to grow, as do gravity, precipitation and animal interference.

As cell walls pull apart, opening up the fracture line, the AZ cells on the tree side close off the opening wound by depositing protective materials such as tyloses, suberin and lignin. This strong protective boundary zone seals off the leaf scar, defending the tree from the cold, as well as from diseases and pests.

With the sealing-off process completed, the leaf can now be safely shed.

While there is still much to learn about abscission, we can see that leaf fall doesn’t just happen, but rather is a carefully coordinated series of complex chemical processes—which would be controlled by the plant’s genes. Researchers have now mapped out a genetic pathway, or ‘signaling cascade’, behind abscission in the common laboratory plant Arabidopsis (water cress).[5] They have identified that there is a key network of genes that code for proteins in a sequential manner. Each step of the cumulative processes that make up the cascade is dependent on the one before it.

This presents a challenge to the evolutionary paradigm—because if just one of these steps in the signalling cascade is absent, the abscission process doesn’t work. (In addition, how did evolution produce genes that code for enzymes that can digest themselves?)

All-or-nothing genetic cascades[6] don’t fit the claimed step-by-step evolution story, but rather fit with the Bible’s account that plants were designed by a super-intelligent creator—God. He designed them to fit the seasons He made too (Genesis 1:14, 8:22; Deuteronomy 11:14). Autumn, and its colorful cascade of leaves, are no accident!


 

[1] Coder, K., Falling tree leaves: leaf abscission, University of Georgia Daniel B. Warnell School of Forestry Resources Extension publication FOR99–025, December 1999

[2] For more on the wonders of photosynthesis see Sarfati, J., Green power (photosynthesis) God’s solar power plants amaze chemists, Journal of Creation 19(1):14–15,
2005.

[3] E.g. pectinase and cellulase.

[4] I.e. the middle lamella. Calcium bridges across cell wall materials are also removed.

[5] Cho, S., Larue, C., Chevalier, D., Wang, H., Jinn, T.-L., Zhang, S., and Walker, J., Regulation of floral organ abscission in Arabidopsis thaliana, Proceedings of the National Academy of Sciences USA 105(40):15629–15634, 7 October 2008.

[6] An example from animals and humans is the blood
clotting cascade, with over a hundred factors or steps now known to make up the sequence. See: “Irreducibly complex: The clotting cascade”, Creation 33(3):15, 2011.

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