August Plant Fact

The shape of leaves and why leaves developed into the huge variety of shapes and sizes that we see on a daily basis has always been a fascination to me.  For example, why are some leaves large and shaped more or less like a hand while others are divided into small leaflets that are connected via a ‘temporary stem’ called a rachis?  Most leaves are simply one large flat plain of leaf tissue that is attached to the stem via an appendage at the base of a leaf called a petiole; this leaf arrangement is described as ‘simple’.  Those leaves that consist of small leaflets that in turn are attached to a central rachis are called compound leaves.  To make leaves even more confusing, some plants have a main central rachis, from which secondary rachises and their associated leaflets arise, which is called a twice compound leaf!  A prime example of a simple leaf would be that of a Tulip Tree (Liriodendron tulipifera, pictured below in fall color) compared to the compound leaf of a Black Walnut (Juglans nigra) and a twice compound leaf of a Kentucky Coffee Tree (Gymnocladus diocus, pictured below).  Obviously, each plant developed a different strategy for capturing energy from the sunlight in its quest to create sugars from CO2 and H2O.  However, there were numerous additional developmental pressures that were influential on leaf morphology, beyond simply strategies for capturing the energy from the sun.  What complicates the issue is that leaves did not follow simply one path of development.  Rather, there were numerous paths of leaf development occurring simultaneously over a several hundred million year period!  Some of these developmental pressures include the regulation of the leaf temperature during exposure to the sun, reducing herbivory (predation by herbivores) and minimizing water loss during dry or windy periods. 

Tulip Tree
leaf2
Tulip Tree
Kentucky Coffee Tree

Water is often a limiting factor for leaves, both for conducting the process of photosynthesis and for maintaining the health of the cells in the leaf tissues.  Developing different strategies for transferring water to all portions of a leaf resulted in the development of a variety of different styles of leaf venation and leaf shapes. If you have ever studied a leaf, you will notice that there appear to be ‘suture lines’ that travel throughout the leaf, which are referred to as leaf veins.  These veins are in essence the pipes that transport the water that moves up from the roots and stems to cells within the leaf, while simultaneously transporting carbohydrates created in the leaf tissue back to the stem and then down to the roots!  Obviously, the more veins that a leaf contains, the greater the efficiency of the leaf for transporting water and carbohydrates!
Ferns, which date back upwards of 350 million years ago (MYA), have leaves with a midrib down the center of the leaf and a network of veins radiating out from this midrib.  Dating back to  around 300 MYA, ancient gymnosperms, such as Cycads, Gingko (as pictured below in fall color) and Pine Trees developed a more simplified leaf venation, whereby numerous veins are distributed in a parallel fashion and run directly from the basal end of the leaf near the leaf petiole, to the margins of the leaf.  By contrast, Angiosperms or the true flowering plants (around 140 MYA), reversed the simplification of leaf venation and feature a hierarchical series of veins, much like the circulations system within humans; the veins or vascular system is initially relatively large near the leaf base, but it proceeds to continually divide into the smaller and small veins throughout the leaf.  This ensures that all parts of the leaf receive adequate water and a more efficient transfer of carbohydrates into the vascular system.   Angiosperms also possess very high vein densities.   More primitive plants have an average of 2 mm of vein length per mm2 of leaf surface.  By contrast, angiosperms average 8-10 mm of vein length with some plants reaching as high as 20 mm! 
Another aspect that impacts the shape of leaves is how they grow and expand during the spring.  For most ferns and gymnosperms, the region of growth is only along the margin of the leaf, not throughout the leaf.  This type of growth severely restricts the potential variation in leaf morphology.  By contrast, expansion and growth of Angiosperm leaves is diffused throughout the entire leaf!  Combined with an improved circulation system, the angiosperms are capable of a far larger diversity of leaf shapes. 
Reflecting back on the initial question of why some plants merely have simple leaves while others have compound leaves – most likely this is not the result of one environmental pressure, but many pressures. However, the efficient transfer of water and carbohydrates is certainly a key pressure.  Plants with large leaves are typically – but far from always – associated with moist, shady locations.  Plants that experienced a prolonged period in which there was a need for increased efficiency of getting water to the leaves or a carbohydrates to the roots would have pressure to develop a more efficient leaf strategy than transferring the materials across a long network of smaller veins, such as would exist in a large leaf.  In compound or twice compound leaves, a greater proportion of the true leaf tissue is close to the larger main vessels within the rachis.  As a result most of the major ‘piping system’ is closer to the regions of photosynthesis and the plant can more expediently transfer water and carbohydrates!  Once again, fascinating!

leaf

leaf

 

Ginko