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Helianthus annuus hybrid
This is the backlit leaf of a giant sunflower growing in the Food Garden here at UBC Botanical Garden. Despite the general cloudiness of this past summer, a few of these plants have easily exceeded 3.5m (~12ft) in height.
What is immediately obvious is the network of veins in the leaf. While the veins are essential for nutrient and water transportation, in a number of leaves they also help to provide a structural skeleton. Each vein consists of xylem (water-conducting) tissue and phloem (sap-conducting) tissue surrounded by a protective wrapping of structural tissues, including collenchyma. In the spaces between the veins (and bounded by the upper and lower epidermi of the leaves) is the tissue called parenchyma. In leaves, parenchyma cells are packed with chloroplasts, the organelles that are responsible for photosynthesis. It should be noted that other parenchyma cells may have different functions elsewhere in the plant, such as storage or secretion. This is why the chloroplast-laden parenchyma of leaves is sometimes referred to as chlorenchyma, so as to better reflect the specific purpose of this tissue.
I don't mention photosynthesis often enough on BPotD, considering the fact that nearly all complex life on Earth depends on this process in one way or another. The evolution of photosynthesis approximately 3.5 billion years ago and subsequent alteration (with some lag time) of the Earth's atmosphere to the present day concentration of oxygen in the air permitted the evolution of complex life.
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