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Nature's Palette

The following is my comment on reading Nature’s Palette, 2007, University of Chicago Press (cover price $US 35.00, hardback, Amazon price as of 7-27-16 $US 17.97 in paperback) by David Lee, professor, Florida International University, and Director, Kampong of the National Tropical Botanical Garden, Miami


This was the book mentioned here in a posting by Corey W. on  6-22. I acquired a copy and have been perusing it for a couple of weeks. As Corey said, it does not discuss bullation or specifically any of the other questions we popped at that time. But it contains a large number of excellent pictures and is chock-full of interesting bits about plants and botanical science generally. It is possible that Dr. Lee is actually a member of this list, although I don’t recall seeing his name before.


This is a science book, but contains enough first-person anecdotes to make it rather like reading a letter. The pictures, as I mentioned, are abundant. There are lots of aroids. Many of them are taken in and around Dr. Lee’s home in Miami. So it might connect to those of our members who inhabit South Florida.


Most of the book has to do with understanding how different plants manifest colors. In leaves. In stems. In fruits. In roots. But my chief interest was in trying to answer a couple of my questions, so I concentrated on green, especially in leaves, and how chlorophyll is distributed. As one might expect, green is a dominant color theme.


I did run across two particular items. Both relate somewhat to my interests. The first is a diagram (page 98, see attachment, included here without permission) of a leaf cross section. It dates from 1918. Reading Dr. Lee suggests to me that not much has really been added to scientific knowledge since then.


What we see is light rays coming down through the outer skin layer, through what is called the palisade layer, and then partly scattering off this or that leaf element. In this case, most of the light rays get absorbed, although, as we know, the green portion of the light does not get absorbed very well, which is why we see green in leaves. The important thing from my point of view is the scattering.


Next there is a related quotation from pages 128 and 129:


Light strikes the upper surface of the leaves of this tree [his example, the umbrella tree]. It enters into the leaf and passes through the palisade layer with very little scatter. Since the chlorophyll is in packets (the chloroplasts), much of that light passes through this layer via the central vacuole into the spongy mesophyll, where it is strongly scattered by reflection due to due to the differences in the refractive indexes of the cell walls and air spaces. This provides the path-lengthening effect discussed in chapter 4 [earlier in this book]. Some light is scattered through the palisade cells, adding to photosynthesis, and then passes out of the surface. This makes the leaves look dark green to us.


OK. So there are a few technical terms in there, but not nearly as bad as it could have been for the lay reader (me). The interesting thing for me was that photosynthesis seems to happen even with scattered light beams that are not coming down perpendicular to the leaf surface.


This seems at odds to the effect known as phototropism. That is, the movement of leaves in response to light whereby they attempt to situate themselves in such a way as to gather the most light coming directly down onto the leaf surface. Light coming in sideways is not to the liking of leaves, so they gin up some chemicals that turn the leaves so they can take better advantage of the light. Light coming from below the leaf is especially problematic for them. Unless the leaf can turn itself back to receive light coming in correctly that leaf will be sacrificed in favor of new ones.


And yet, Dr. Lee indicates that light can be used for photosynthesis even if its direction is random.


My guess is that scattered light will only work for photosynthesis if it gets itself in a position to mimic incoming perpendicular light beams. Leaves are very structured and my thinking is that this structure is there for a reason. A new question arose as I thought of this problem: What is the role of orientation in single-cell photosynthetic species such as blue-green algae? Either we now have cells that work efficiently at any orientation to the incoming light or we have cells that attempt to situate themselves in a geometry of some kind that works to maximize energy capture. It suggests that single-cell blue-green algae cells would have a sort of “head” and “tail” arrangement as they float otherwise freely.


And still no word about photosynthesis in green petioles and stems.


In any event, some readers in this space might want to get a copy of Nature’s Palette. Always some new things to learn.


Ted Held


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