The  leaf

Ask one hundred persons, young and old from any continent, what the pictures herebelow represent, and most people will instantly answer: "Leaves,of course!". Everybody knows them, but what are they indeed?
 

 
Leaves, a protrusion of the stem, usually green and flat, but sometimes also quite variable in shape, size and even color, are object with a characteristic inner structure and own metabolic functions. During their formation and growth they develop specific features with which they efficiently produce oxygen and organic matter (sugars) from carbon dioxide and water in the presence of light, a process called photosynthesis. We seldomly realize but almost all life on land, including our existence as humans, eventually depends from the presence of leaves. In the architectural plan of the plant leaves all varieties of 'tricks' can be recognized, thanks towhich plants can survive in the most extreme areas on earth. In the next pages we will dive into the microscopical world of leaves and get an impression on how this fantastic architectural piece is built and how it works.
 
As an introduction a quizz question: "What are common points between a leaf, a dish antenna and the Dutch polder landscape?"
Read the text here below for the answer
 

 
Leaves appear in a large variety of shapes, but they are generally flat and have an upper and lower side. Thanks to their relatively large surface they can, a bit like a dish antenna, better perform their main function, that is to capture energy and convert it into another signal. In the case of leaves, light is captured and this energy is utilized for the photosynthesis reactions that occur inside the chloroplasts, which pigments gives them the typical greenish color. In general leaves have a bifascial structure (upper and lower face are different) which corresponds to the orientation to the sunlight (a similar directionality toward the source is observed in dish antenna's too).
Furthermore, striking for most leaves is the presence of a complex vein system (vascular bundles). In higher ferns and dicots (for examples ivy en petunia) a large midvein branches into smaller reticulate or feather-like veins. Leaves of monocots on the other side, exhibit parallel veins interconnected by very small veins (e.g. papyrus). Thus, to a certain extent, the venation pattern resembles the small canals in the typical Dutch polder landscape, and similarly they both serve the flow of fluids. One of the differences between both systems is that in each vein two separate streamings of sap flow, and this in opposite direction: a influx of water and minerals tothe leaf and an efflux of sugars to the remaining parts of the plant.
 
Authors:

• Content of the lesson:J. Derksen, M. Wolters-Arts, W.L.P. Janssen and E.S. Pierson
• Images, database: W.L.P. Janssen, D. van Aalst, B. van der Linden, J. Hiddink, H. Geurts, E.S. Pierson
• Plant material: G. van der Weerden (Botanical Garden Radboud Universiteit Nijmegen)
• Web-structure / lamascripts: R. Aalbers
• Correction of the English webtext (partly): D. Bonsack
• Contact: E.S. Pierson (Liesbeth)