Signs of Summer 5: Leaf Shapes (and great questions!)

Photo by D. Sillman

A couple of weeks ago I got an email from a Seventh Grade student named Gordon. Gordon was doing a Life Science project that involved coming up with a question that could not be answered by simply doing a Google search. Gordon’s question was, “What determines each tree’s type of leaf and their structure?”  Gordon found my essay out on the Virtual Nature Trail about leaf shapes and strategies and thought that I would be a good person to possibly answer his question. Here is my answer! My compliments to Gordon for coming up with such an interesting question and also to Gordon’s science teacher for coming up with such a creative and meaningful project!

 

Gordon:

What a great question!

Leaves are incredibly important to any plant since they are the organs where photosynthesis occur. Since the shape of a leaf is controlled by a number of very specific genes most scientists feel that natural selection and evolution must have played a role in determining leaf shape. It is not clear, though, what exact factors in a tree species’ ecological and evolutionary environment were the determining, natural selection variables for leaf shape.

Looking at the extremes of leaf shapes (needle shaped leaves vs. broad, flat leaves) (and this is what I wrote about in the on-line essay that you found), it is pretty clear what evolutionary factors were in play: needles can withstand the very dry and very cold conditions of winter and, thus, persist for many years on a tree. Broad leaves cannot handle the dryness and freezing conditions of winter, but they are more efficient photosynthesizing organs! So a tree balancing its energy demands either makes a leaf (a needle) that lasts for several photosynthesizing seasons but generates, each season, less sugar from photosynthesis, or it makes a single season leaf (the broad leaf) which generates a lot of sugar in one season and then “throws the leaf away” in the Fall! You would expect in colder or drier environments the “needle leaf” solution would work best for a tree!

Looking at overall leaf size, it is also pretty clear which selection factors are at work: in any type of stressful environment (very cold, very hot, very dry, low nutrient, or high salt conditions) trees tend to make very small leaves. These leaves, while less efficient in photosynthesis than large leaves, match the energy balances required for the tree to survive under these very stressful conditions.

All of the more subtle differences in leaf shape, though, are much harder to explain! There are a few really interesting hypotheses, though.

First of all, when a leaf develops on a tree its tissues (where the cells are that contain the chlorophyll that accomplish photosynthesis) develop and grow around the leaf’s vascular tissues (its “veins”). These leaf veins bring water to the photosynthetic cells and take away the sugars that they make in photosynthesis. Overall leaf shape seems to be correlated to the energy efficiency of this water delivery/sugar transport system! Each photosynthetic cell has to be close to a leaf vein! Lots of different shapes can “solve” this energy requirement successfully!

Also, when sunlight hits a leaf LOTS of heat is generated! So the leaf, when it photosynthesizes must dissipate this excess heat out to its environment. Some of the sculpting of the leaf edges (the serrations and deep invaginations into the leaf mass) may be related to solving the problem of efficient heat dissipation.

Here are a couple of types of oak leaves that illustrate these ideas: (the images are from Wikivisual and are listed under Creative Commons usage rights):

Chestnut oak leaf

You see the leaf veins in each type of leaf, and you can imagine the leaf forming by growing the leaf tissue around those veins. The close proximity of the leaf tissue to the veins sets up a very efficient water delivery/sugar exporting system! The deep invaginations of the pin oak leaf, though, means that although there is less leaf tissue for photosynthesis, there is more efficient heat dissipation from the leaf! The balance between overall photosynthesis rates and heat dissipation may be the factors determining the subtle differences in the shapes of a particular

Pin oak leaf

tree species’ leaves! Not surprisingly, chestnut oaks tend to grow in colder environments, and pin oaks tend to grow in warmer environments!

Thank you for your wonderful question! I plan to use some of my answer in my weekly ecology blog sometime this summer! I will be sure to give you credit for the question! You can find my blog at https://sites.psu.edu/ecologistsnotebook/

Keep enjoying science!

Dr. Hamilton

Department of Biology

Penn State New Kensington

There are a few other possible variables that might be important in some leaf shape natural selection systems. Plants growing in environments with low light levels might show evolutionary patterns for shapes that optimize light reception. Other plants under intense herbivore pressures might show selection patterns for shapes that resist that herbivory. Leaf shape might also be subject to the complex constraints of biomechanical factors and overall structural integrity of the leaf. There are also a number of hypotheses that consider leaf shape the consequence of selection for genes that code for completely different features of the tree (like flower shape, for example). The impact of these genes on the development and shape of the leaves, then, is just an inadvertent tie-in to the actual selection for that different factor!

I can imagine some experiments that should be conducted to explore this question. For example, do the leaves of a northern red oak (which have very subtle edge invaginations) photosynthesize better than the leaves of a scarlet oak (which have very pronounced edge invaginations) in cooler environments? Or, do those scarlet oak leaves photosynthesize better that northern red oak in warmer environments? Or, do these very different leaf shapes have no effect on their efficiency of photosynthesis? Experiments like these could help to focus the evolutionary discussion of leaf shape and help to define the significance of its possible natural selection variables.

 

 

This entry was posted in Bill's Notes. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *