In fall. When leaves fall..

It’s finally fall in California. And the leaves are falling. I couldn’t help but ask myself some questions about why and what causes the leaves to fall. In addition, they were changing colors as well, so I wondered if they changing colors would reveal a changing morphology or indicate a predominantly compositional change.

So I took the following leaves and put one under the foldscope. Since they were half and half, I was hoping catch a spatial variation in the same leaf.

I spent quite some time looking at the leaves and my conclusion is that there is no morphological difference between the green and yellow parts. See for yourself below.

The left side is the green and right is the yellow. Based on my observations, I’d conclude that changing color is a predominantly chemical change (pigments is my guess) and has negligible morphological effects, at least at short timescales. Perhaps later, desiccation would lead to buckled cells.

So to the next question. How do the leaves fall. I observed something interesting on the stems. Here’s a small bit cut out and mounted.

If you look carefully in the above picture, you will observe two dots that look like eyes. I suspect that they are vessels or veins that transport liquid to the leaf. So now I can start making a hypothesis. When the human body becomes cold, it cuts off blood supply to exterior parts as a survival mechanism- perhaps plants respond in the same way. So, the indication to changing color, could be coming either by reduction in water/nutrients through these vessels, which would trigger the leaves to change color and also create desiccated ‘joints’ that would easily break under the slightest mechanical force (wind/gravity).

Here’s the vessels under the foldscope. (Top-view)

And a side view indicating that they run through the entire stem.

And now for some fun videos.

The first one shows how easily the leaves break. I have to barely touch them. And if you look closely, you’ll see the ‘eyes’ or the vessels that I’m now convinced are the water/nutrient pipeline.

Now watch the same breaking for leaves that are partially green. I almost have to tug at them to free them. Notice the ‘eyes’ are not as easily visible.

The last aha moment. The base of the stems in the partially green leaves is still moist and so a darker color. You can barely see it. However you can clearly see the stem and the vessels in the dried versions as they have desiccated and are more yellow.

So I think my hypothesis of the plants cutting supply off has some merit.

I’m quite sure there is a more complicated story behind, but I’m willing to bet that we understood in broad strokes the gist of how and why leaves fall in fall, by just observing closely.

And isn’t that just a wonderful way to learn?

5 Comments Add yours

  1. laksiyer says:

    @Saad. You are absolutely right and this is a nice way of showing the abscission zone. There is some more detail in this recent paper.

  2. Saad Bhamla says:

    That is a cool article @laks. So its more like get off leaf/shedding instead of falling off.. : )

    I think the abscission cell layer is far more interesting.. now that I know of it, i’ll try to reimage my sample and see if I can visualize it.


  3. Y.S. says:

    Hi Saad,

    This is beautiful. I’m not a scientist these days, but I remember learning (in years past) that plants will cut off water and nutrient supplies to those leaves which take too much energy to supply, especially when reserves are running low, in order send that energy elsewhere. That’s what’s happening when you see a plant with completely dry, dying leaves very close to very young new leaves or fruit. So that would support your hypothesis (vis-a-vis the analogy with the human body).

    Have you foldscoped the leaves themselves? It would be interesting to know if you can see any veins in the green zones vs the yellow zones, and whether they’re different…


    1. Saad Bhamla says:

      One of the first images is of the leaves. I didn’t see much difference on the same leaf in the yellow/green regions, but I suspect a drier leaf could be different due to excessive desiccation.

  4. Manu says:

    @Saad: Incredible observation; and what a great finding. Something in plane sight can be so perplexing.

    To further deepen the mystery and point to one more phenomena; where plants on purpose kill cells at a specific location is when leaves are infected with plant viruses. In order for the infection not to spread; plants will kill an area surrounding the infection; leaving the typical marks we are used to noticing with a plant pathogen.

    Here is a foldscope post from a student in one of my classes:

    As you will see in the comment of that post; Laks taught me about the apoptotic response.. now the question is; could these two phenomena have any links?


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