Plantevolution.info

A simple question with a simple answer: sunlight. The search for this explanation is an interesting story of two trees that searched their way to more light. This quest for finding an explanation does not only give an answer to the difference in form, but also to the difference in growth and development of the trees and leaves of pear and apple. The story starts in China and ends in our orchards...

Each form in nature has a function. This also counts for the form of fruits of pear and apples. To figure out the function of the form, one has to search for clues. These clues can be found by making an analysis of the mode of life of the plant. With fruits like apple and pear however, the fruit is the result of many years, often many centuries, of selection of these characteristics most interesting for mankind. To determine the function of the form of apples and pears, we have to go back to the “prehistoric” wild apple and pear. These wild apples and pears can be found in mountainous region in north/west China, near Kazakhstan. Wild apples are apples that resemble our ornamental apples that we plant in our orchards as pollinator. We consider apple trees as trees, although it are rather bushes that we manipulated to become trees. The wild apple tree is a spherical/globular bush that can be found in the valleys of the Tian Shan mountains. The wild pears can also found back in the same area, but then on the more lower situated slopes of the mountains. The difference in the habitat in the mountains is our first and most important clue for our search for the function of apple and pear.

Each tree wants the same: capturing as much light as possible to grow. We want the same thing when we place solar panels on our roofs: capturing as much light as possible to make electricity. When we place solar panels, we do not place them horizontally on our roof, but place them in a specific angle towards the sun (except at the equator where the optimal position is horizontal). In Belgium and the Netherlands for example the optimal angle is 36° towards the south. If we would construct an artificial tree, we would end with a tree in the shape of a Christmas tree: a tree with leaves positioned optimally towards the sun and sky. (except at the equator where the optimum would be a tree with a horizontal flat shape tree crown. This is almost the case for the trees growing in the Savannah.) The side of the tree directed to the North (in the northern hemisphere) does capture less light, but this is compensated by the south side that has the optimal orientation towards the sun. An apple tree however does not have the shape of a Christmas tree. The shape of the apple tree resembles that of an umbrella. Why doesn’t an apple tree have the shape of a Christmas tree? The answer to this question is simple: trees do not stand alone, but are most of the time part of an ecosystem, a wood or forest. In a forest, the trees are competing for light. An umbrella shape tree captures more light in a forest then a Christmas tree shaped tree. The umbrella shaped tree can capture light that would normally go to the neighbouring tree, giving him an advantage. And since the tree that can capture more light will also grow better, it is quite reasonable that trees in a deciduous forest have most of the time the shape of umbrellas. The same accounts for the fruits of these trees. The largest part of the life time of fruits, the fruits are green and also convert solar energy in “growth” energy. The most efficient shape for fruits for doing this in a forest, is again as they have the shape of an umbrella. Fruits like apples however, change their position during their development. In the beginning they are in an upward position, and later they go to a hanging or downward position. The optimal shape in this case is not the umbrella shape, but a round shape. An apple is therefore round because it wants to capture as much light as possible.

Pear trees do not have an umbrella shape, but have a shape that can be situated between that of an apple tree and that of a pine tree. The pear tree shape approaches more the ideal shape for capturing light: “the Christmas tree shape”. How to explain this difference? The explanation brings us back to the origin of the apple and pears: the Tian Shan mountains in North/West China. The wild pear trees/bushes are predominantly found back on the slopes of the lower mountains. Trees on a slope have less competition for light. The trees that grow lower on the slope do not steel light, and therefore the ideal form of these trees is not the umbrella shape, but the “pear” shape. For the explanation of the fruits, we follow the same way of thinking as we did with the apples. The fruits have taken the same form as the trees because the fruits also capture more sun in this way. Even if the fruits are in the beginning in an upward position, this position changes fast, making the pear shape the most interesting shape for capturing light. This is the reason why the neck of the pear is not round as with an apple, but cone shaped to capture more light. Light again determined the shape of the fruit.

The link between the sun and the form is very clear, but does not give an explanation why the structure of the leaves and fruits are different. Pear trees have more solid/firm leaves compared to apples. Pears are harder than apples and the peel is clearly different. This difference cannot be explained by the optimisation of capturing light. Nevertheless the main clue for finding the explanation for the form of fruits is also the main clue for the explaining for the difference in building structure: wild apple trees mainly grow in the valleys and pear trees on the slopes. As pear trees are situated on the slopes, they are also more exposed to the wind. Plants protect themselves against evaporation/losing to much water. The most energy efficient way for plant for doing this, is by growing hairs on the leaf surface. On the surface of apple leaves a lot of hairs (trichomes) can be found. These hairs hold a thin layer of air that prevent the plant of losing to much water do to evaporation. Pear leaves do not have hairs on their surface, but instead have a more firm/hard leaf surface. This option to reduce evaporation costs more energy and building blocks. Why did the pear tree chose this strategy? Because pear trees are located on the slopes, they are more exposed to the wind. At higher wind speeds the hairs on the leaves cannot hold the stable air layer at the leaf surface that prevents the evaporation. To overcome this problem, the pear tree evolved and leaf surface became thicker and firmer. The hairs on the leaves became redundant as the thicker leaf surface took over their function (only young leaves still have hairs on pear trees). The same accounts for the fruits. The pear and apple fruits structure evolved differently because they also had to overcome this problem of the influence of the wind on the evaporation. For pears this resulted in harder fruits and a different ratio and development of lenticels and stomata. Wind was the driving force for the difference in building structure between apple and pear. Together with the sun, the wind made apple and pear trees as they are as we know. Apple and pear had a common ancestor, but as they started to grow in a different areas in the landscape, the shape and structure changed. Now we plant both apple and pears in the same region and take little account to the geology of the region. The only evolution the trees in our orchards now undergo is the direction we determine, sometimes still in function of light and wind, but not steered anymore by light and wind.

The natural habitat of pine trees is higher grounds in the mountains and more northern countries. The shape of pine trees resembles that of pear trees. The explanation of the tree shape of pine trees is therefore similar to that of pear trees: less competition with neighbouring trees resulted in a more optimal shape for capturing light. (For trees growing high in the north, the angle of the sun makes it difficult to capture light. In this case the cone shape wins from the umbrella shape due to the constrains of less light intensity. The competition for light in the forest had a different outcome in these northern regions.) The shape of the fruits however is the opposite form of that of a pear fruit. This can be explained by the fact that pine fruits are standing most of their life span in an upward position. Again the form of the fruit can be explained by the theory of efficient light capture/use.

The question on the form of apples and pears was a question that I first asked when I started doing research in fruits. The theory above took form 6 years ago, and since then I have presented it several times when giving a talk on fruits. The theory was build by reading many articles. Not only articles that sustain this theory were important, but also articles that refuted alternative hypotheses. Sometimes, the latter were even more important. Making a choice/selection which articles to refer to is therefore difficult. All researchers on plants must be thanked. Furthermore the list of articles is still growing as this is not a static theory. References are also normally published to sustain a theory. In this case the sustainability is put to the test by the readers. If the theory proofs to be wrong, adjustments will be made and published on www.plantevolution.info.

The originality lays in coupeling the origin of the trees in China and the habitat of the trees to the shape of the trees. Coupling sun and light to the structure and form of these two fruits. The ideas presented here are hypotheses. These hypotheses cannot be proven and might even never be proven. It is probably only one of many explanation for the form of apples and pears. The simplicity of the theory and because the theory does not only give explanation to one question, but gives answers to many questions, makes this theory interesting. Even if it is not the correct theory, it is at least a nice story…

ORIGINALITY PROTECTED BY

BOIP i-DEPOT

ORIGINALITY PROTECTED BY

BOIP i-DEPOT