Tuesday, 06 December 2016 17:09

The Orchid Leaf

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The leaf is where water, carbon dioxide and sunlight combine to create a glucose that is the orchid's food source.  Healthy leaves are desired for this process to function.  Unhealthy leaves can lead to problems and even the loss of the orchid.  This section will detail how the orchid leaf functions, and some common negative issues of unhealthy leaves and how to prevent these from occurring.  Sun burn is the biggest cause of leafs failing and the topic of how much light is too much light will be covered in it's own discussion area.


This topic will be the feature of the May 2017 Napa Valley Orchid Society's Newsletter.


Leaves and leaf structure

Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells.

The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf.

Water enters the root and is transported up to the leaves through specialized plant cells known as xylem vessels. Land plants must guard against drying out and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through the stoma (the singular of stomata), flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal of water is also lost. Cottonwood trees, for example, will lose 100 gallons (about 450 dm3) of water per hour during hot desert days.





The Orchid Leaves Orchid leaves are just as varied as the flowers. It can be broad, thin, succulent, cylindrical, tiny and even as huge as more than a meter, i.e. three feet in length. Most of the orchid leaves occur in shades of green, blue and grey. There is however, a group of orchids that have leaves that are shades of grey, green, red, brown, silver, bronze and even in copper tones – the so-called jewel orchids. Orchid leaves can grow in various ways: fan shape, at intervals ranging from a few to several centimeters in between. Whichever way it grows, it reflects the adaptations of the orchid to its environmental conditions. For example some Vanda orchid species grow in shaded areas and thus their leaves are broad, flat or pinnate for maximum exposure to sunlight. The other example that warrants mention here is the Brassavola species which grow naturally in tropical regions in harsh sunlight. Their leaves are fleshy and pencil shaped to as to expose the minimal surface area of the plant and retain moisture.




Stomata (sto-MAH-tah) are pores on the lower surface of the leaf epidermis through which the plant breathes. The stomata are mostly closed during the day to prevent water loss by transpiration and open at night when temperatures are lower and humidit y is higher. This means that orchids are not good candidates for foliar feeding. If specialty foliar sprays such as those containing minor or trace elements designed to be absorbed through the leaves are to be used, they are best applied to the underside s of the leaves in the predawn hours.






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