The orientation of a surface, such as a leaf, influences the energy balance of the surface. The heat load of a leaf can also be influenced by air movement and by evaporation (transpiration) of water from a leaf. This will be discussed in more detail in Lesson 12. A small object will tend to be close to air temperature because of the limited boundary layer (ease of air flow around the object). A small leaf in a sunny and stressful environment may have less heat stress than an otherwise identical large leaf.

Figure 11.7 shows some leaves of the desert scrub oak (Quercus dumosa), a kind of oak that grows in California. In the arid West numerous oak species exhibit adaptation to the climate-some are deciduous (lose leaves seasonally); some are evergreen (live oak has leaves all year). The desert scrub oak variety, Turbinella, is an evergreen oak tree. The leaves stay green year round. It grows in places where it never, or very infrequently, freezes, often a hot and harsh environment. These leaves are thick, perhaps waxy. The oaks of the bottom of the Grand Canyon are similar.

Fig. 11.7 Desert scrub oak leaves

Oak trees are a headache to botanists. A botanist does not care to tell you what kind of oak tree you are dealing with. If you ask a botanist, "What kind of oak tree is this?" He may say, "Oh, that is pretty close to a white oak," or "That is pretty close to a pin oak." A botanist seldom says, "That is a white oak,""That is a pin oak," or "That is a red oak." Why? Because that is true. It is just "pretty close." Many oak trees cross with each other freely, so you never really know what kind of an oak tree you have. If you are a smart botanist, you will say, "It's pretty close to...."

We will look at an oak tree that someone thought was a Douglas oak, or a blue oak of northern California. Its leaves fall off in the winter. At the top of the picture is the desert scrub oak (Dumosa oak). The blue oak (Fig. 11.8) has generally larger leaves.

Fig. 11.8 Leaves of the Scrub Oak (top), Blue Oak (bottom), and their hybrid (center).

There is a difference in leaf size between blue and scrub oak. When these two trees cross, all the leaves are intermediate. They are kind of in between size and in between shape.

After a generation or two, the hybrid begins to segregate according to the environment. Some of the leaves could be just as big as on the blue oak. Some leaves could be just as small as on the scrub oak. The distribution of those varieties turns out to be the interesting thing.

Benson (1967) looked at the size of leaves and where they were found. In the middle of a valley in California there is a little hill, not a mile across, covered with oak trees. On the southwest side the leaves are all small; on the northeast side they are all big. In between they are intermediate. So there is something about the environment that is selecting for trees with the biggest leaves on the northeast size and the littlest leaves on the southwest side (Fig. 11.9).

Fig. 11.9 Leaves on the NE side of a hill are larger than those on the SW (from a study of a hybrid of Quercus douglassi x Q. rurbinella by Benson et al., 1967.)

What is it about the environment that is selecting this? It is thought it has something to do with an ideal leaf size for the environment. Leaves do many things to adapt to their environment. Animals hide. They go to a place that is warmer or cooler. Plants do not have that luxury. They have to do something else. Plants adjust their leaves. They move their leaves to a different angle to the sun, adjust their stomates, and maybe even change their color. The main things is that the plant has done something so that the leaves are suited to its environment.

You can see this when you buy a decorative plant from a store. A weeping fig is a dramatic example. People bring it home, set it in the corner of their front room by a window, and the leaves all fall off. If things go well, it grows new leaves and then it is happy there. Eventually, the people that live in the house want to change the furniture, so they move the furniture and the plant. Then the leaves all fall off. Sometimes when all the leaves are off, the plant dies.

There has been a lot of money spent by the horticultural people who sell decorative plants because they get a lot of complaints: "I bought this weeping fig, and I took it home and the leaves all fell off, every one of them!" They say, "Well, take good care of it. They will grow back." But when they grow back, they are a different size, shape, and thickness than before. If they are healthy, their thickness, size, shape, color, and the number of stomates per centimeter on their surface will be ideal for the environment that predominated when it started to grow the new leaves.

All plants do this to some extent. Raunkiaer (1934) noticed that leaves tended to be larger or smaller depending on their environment. He went through the herbariums of Europe and classified leaves. From smallest to largest, he classified them as: leptophyll, nanophyll, microphyll, mesophyll, macrophyll, and megaphyll (Fig. 11.10).

Fig. 11.10 A classification of leaves according to area for leaves with typical elliptic shape. The smallest leaves are leptophylls (<0.25 cm2) and the largest are megaphylls (>1640.25 cm2) (Raunkiaer, 1934).

Raunkiaer discovered that these classifications worked quite well. Each classification is nine times as big as the previous. Then he went through herbariums and looked at all of the dried leaf specimens. He evaluated all of the leaves of basic elliptic form. He characterized them and found that they came from a certain climate.

Little leaves were found in arid places, and big ones grew in cool places. It was much like on the northeast versus southwest sides of the hill in California.

But Raunkiaer could not compare the oak leaf directly to an elliptical leaf with a smooth side. That was something that he could not do without some way of normalizing leaf shape.

Leaves are important. Nothing is more important to a plant than its leaves. Nothing is more important to the efficiency of the plant than the efficiency of its leaves. Leaves need photosynthetic efficiency and water use efficiency, and they need to be able to survive in the area that they live in. Water is not always a factor. Photosynthesis always is.

Does the leaf have to be optimal? It has to be optimal in some way. Plants, to be competitive in nature, must optimize in some way. This is crucial.

There is native cotton in the southwestern United States. It has a very different leaf shape and size than commercial cotton that grows adjacent to it (Fig. 11.11).

Fig. 11.11 Cotton leaf forms include (left to right) super okra, okra, standard and common.

Commercial cotton cannot really compete with the wild cotton. Farmers tried to keep wild cotton from getting started in their fields, because wild cotton was more vigorous than the commercial cotton. It was a weed. It was well adapted to the environment, better adapted than the commercial cotton grown there. Also, the wild cotton harbors numerous insect and disease pests that adversely impact cultivated cotton.

Looking at the data from the oak leaves, is there a difference in what the ideal leaf should be as we go around the United States? If we would look at the environment in Georgia and Alabama, would it call for a different size or shape of leaf than in Arizona? If we look near the ground, in the middle of the canopy, would we want a different leaf characteristic than at the top? Perhaps they should be different.

As we look at wild cotton, we saw that not only were the leaves a different size and shape than they were on the domestic cotton, but that they changed from the bottom of the plant to the top of the plant (Fig. 11.12).

Fig. 11.12 Wild cotton exhibits a range of leaf form on an individual plant.

This was not true for domestic cotton. The wild cotton is twice as efficient as the domestic cotton (water use efficiency). On a basis of photosynthesis per unit area, the wild cotton is more productive than domestic cotton in the Arizona climate.

Some USDA scientists began to look at the different leaf forms to see whether they could get a more efficient plant if the breeders brought leaf shape from the wild cotton into the domestic cotton. When they got the size and shape of the leaves, they found out that it produced exactly the same amount of cotton as it had before. But it did it with less than half the water. So they kept the efficiency up, cut down the plant disease, and used only half the water that they had previously been using. By having fewer leaves and less leaf area, more air circulated through the canopy and reduced some of the disease problems in cotton (Buxton et al., 1974). Occasionally leaf form is revisited in contemporary research (Figs. 11.13 and 11.14.)

Fig. 11.13
Fig. 11.14

If it works with cotton, it probably works with every plant. There is likely an ideal leaf size, shape, other characteristics of the leaf for soybean, corn, every plant, and every climate.

We want to understand, to some extent, what the ideal would be. It is difficult to determine the ideal. To be able to compare leaves we need not only to be able to compare similarly shaped leaves, but to compare all leaves of any shape and have some index for comparison. Comparing leaves of different shape requires normalization to a characteristic dimension.

Table 11.2 Classification of leaves according to size and by characteristic dimension adapted from the methods of Raunkiaer (1934)a.
Leaf Area
One Side
Width x 0.742=Dc
Leptophyll 0-0.25 0-0.33 D-leptophyll
S 0-0.056 0-0.16 D-Le-S
M 0.056-0.12 0.16-0.24 D-Le-M
B 0.12-0.25 0.24-0.33 D-Le-B
Nanophyll 0.25-2.55 0.33-0.93 D-nanophyll
S 0.25-0.52 0.33-0.47 D-N-S
M 0.52-1.08 0.47-0.68 D-N-M
B 1.08-2.25 0.68-0.93 D-N-B
Microphyll 2.25-20.25 0.93-2.75 D-microphyll
S 2.25-4.68 0.93-1.32 D-Mi-S
M 4.68-9.74 1.32-1.80 D-Mi-M
B 9.74-20.25 1.80-2.75 D-Mi-B
Mesophyll 20.25-182.25 2.75-7.38 D-mesophyll
S 20.25-42.09 2.75-3.8 D-Ms-S
M 42.09-87.68 3.8-5.3 D-Ms-M
B 87.68-182.25 5.3-7.38 D-Ms-B
Macrophyll 182.25-1640.25 7.38-22.26 D-macrophyll
S 182.25-378.82 7.38-10.8 D-Ma-S
M 378.82-789.13 10.8-15.2 D-Ma-M
B 789.13-1640.25 15.2-22.26 D-Ma-B
Megaphyll 1640.25-x 22.26-x D-megaphyll
S 1640.25-3409.31 22.26-31.5 D-Mg-S
M 3409.31-7102.11 31.5-43 D-Mg-M
B 7102.11-x 43-x D-Mg-B
a Leaf-dimension classification is directly derived from the leaf-size classification by Raunkiaer (1934) for elliptic leaf form. The elliptic form does not constitute an ellipse which has D = width x 0.807, but the dimension is found as D = width x 0.742. Each class is divided into three groups: small, medium, and big, as suggested by Raunkiaer, with the areas for each division chosen by the author as consistent with the original class size divisions.

b S, M, B (small, medium, and big) are class divisions suggested by Raunkiaer (1934) but divided (values chosen) by Taylor (1974).

c The characteristic dimension for Raunkiaer's leaf outlines is width x 0.7420, after Parkhurst et al. (1968).

A characteristic dimension can be given for a leaf of any shape. No matter what the shape of the leaf is, at the same environment, it has the same characteristic dimension. This turned out to be interesting because growing in the same environment, near St. Louis, were these two leaves: an oak leaf and an aristolochia leaf (Fig. 11.15).

Fig. 11.15. An Aristolochia leaf (a) has a smaller area than the Quercus palustris leaf (b). But ecologically the leaf is smaller and more adapted to arid conditions.

Ecologically, the small size leaf is larger. Even though the larger leaf has almost twice the surface area of the small one, the way it is lobed makes it easy for the wind to carry away heat and water vapor. The larger leaf weighs more and has more square inches of surface area, but is ecologically the smaller leaf.

As an exercise, we will determine the characteristic dimension of a bluebell (Mertensia sp.) and a senecio leaf (Fig. 11.16). First for the bluebells, if we come up with a characteristic dimension, the dimension to compare it to a football-shaped leaf, for example, then we will be able to compare all leaves. If I wanted to know its characteristic size, first I could measure the length of the leaf. It is not quite the football-shaped leaf in Figure 11.10, so I will measure its maximum width. Then I will move a quarter or a half inch and measure the width there and write down the values. Move another half inch and write down the width. Record all of these measurements. Now I can find, not exactly the average width, but something that represents the average. In this way I can tell what size of a rectangle it would take to lose the same amount of water or heat. I will determine a dimension that will tell me the size of a rectangular plate that would be representative of this leaf. I will call that a characteristic dimension, and then I can look up the class on Table 11.2.

A characteristic dimension for any leaf may be calculated according to the formula (Taylor, 1974):


Equation 11.1

The formula says that the characteristic dimension of the leaf (D) is equal to the sum of the leaf's measured widths. Moving across the leaf every half inch (or whatever fixed distance width moved) and measuring the leaf's width, created a width called "D". Sum these as you move across. deltaW means how far moved. Then divide by the value on the bottom, taking the square root of each one of those. If the first measurement on the leaf was 4 in. wide, I took the square root and wrote down 2. If the next one was 1 in., I took the square root, and wrote down 1. I added the 2 and the 1 together and added up all of the square roots, then divided the sum of the square roots into the sum of the real numbers. I squared that and it gave me the rectangular width of a flat plate the same mean length as the leaf. That plate would lose the same amount of water and heat. It is the characteristic dimension of the leaf.

Fig. 11.16 A Bluebell (mertensia sp.) top and a senecio sp. leaf

The leaf must be measured at a uniform increment. It can be traced onto a piece of graph paper and, at uniform increments, its width measured on every line in the graph paper. Write down each of those values, add them up, take the square root of each one and add up all the square roots. That is the way the formula works. Divide it out and square the answer, according to the formula.

From a practical standpoint, a value "RD" can be assigned to a specific leaf shape. Then the dimension can be found as the product of "RD" and the maximum leaf width (Fig. 11.17).

Fig. 11.17

We have characteristic dimensions of several leaves. Does this work? For the Panama Canal Zone, to live through the dry season, a leaf should be at most 4 in. wide and about 8 in. long. In the wet season they could be bigger than that. Bananas live through the dry season with leaves much larger than this. How do they survive the dry season when they are too big to be ideal for the environment? They would seem to get so hot that they would use either too much water or they would cook and die.

But the leaves adapt to the conditions. For survival, the leaves are tattered with torn edges dividing them into segments that are no more than 5 inches wide (Fig. 11.18).

Fig. 11.18 Tattering of banana leaves is important to water use efficiency and to leaf survival during the dry season. Click for more information on banana leaves.

The banana leaves that had not torn into segments no more than 5 in. wide had dead pieces and places on the leaf, because they could not survive if they were more than 5 in. wide. So the banana leaves that did not tatter had not made it (Taylor, 1970).

Palm tree leaves, not yet unfolded, still developing in its sheath, are all an intact leaf. As they emerge, they tear. Much like the banana leaves that tear in the wind, palm leaves break into little segments as the leaves emerge (Figures 11.19 and 11.20).

Fig. 11.19
Fig. 11.20

If an unemerged leaf is opened up, it is one solid leaf that looks a lot like a banana leaf. It tears as it emerges. Plants growing in Panama had adapted to do exactly what I thought they should do--stay less than a foot long and 5 in. wide (or it could be narrower than 5 inches and more than a foot long). Their rectangular size came within the limits that we had calculated for the ideal.

Data from Panama indicates that the average size of the leaves in Panama is right at 4 inches (Fig. 11.21).

Fig. 11.21

Some were tiny, and a few were bigger than the average. Most were the dimension on that Raunkiaer classification, Ms, or mesophyll. An Ms-B leaf is 5 cm (2 inches) to 7 cm (2.75 inches) wide.

There is something important to leaf size and shape. Remember leaves can do other things. They can adjust their angle throughout the day. They can adjust the resistance to loss of water and, over time, perhaps color and leaf thickness.

There are a number of mechanisms and adaptations, but the first adaptation of a plant to its climate is leaf dimension.