This week we looked at a few leaves belonging to plants that have adapted to survive the niche in which they found themselves. Waterlilies have adapted to survive floating atop bodies of water, the rubber plant has adapted to survive tropical and equatorial climates, and the oleander has developed advantages for surviving in very dry climates. The Nymphaea (waterlily) leaf has stomata on top of their leaves instead of below to allow for increased air exchange and nutrient exchange; they do not have a defense against transpiration as other plants do in the form of guard cells. The loss due to transpiration is not a primary issue for the waterlily since it has extremely good access to water. The sclereid labeled above is meant as a support for the leaf; it helps tent up the leaf which allows for greater air exchange in the leaf air space and provides flotation for the pad. The cuticle of the leaf is quite thin and helps repel water from the stomata. -Chris Barrett The sunken stomata of the Ficus plant helps the plant retain water. By not being flush with the rest of the epidermis, the stomata allow water vapor to be released yet not be immediately blown away by wind, therefore retaining an amount of water for reabsorption. The hypodermis is quite thick on top of the palisade mesophyll cells, perhaps for protection from intense UV radiation. The sub-stomatal regions are quite large, allowing for greater gas exchange in the mesophyll cells. There are not any trichomes visible on this leaf section. This large open areas in the spongy mesophyll, the sunken stomata, and the thickened hypodermis point to this plant being able to survive very hot, and very sunny climates, perhaps in the equatorial region of the world. -Chris Barrett The adaptations of the Nerium oleander plant have allowed it to survive in very dry climates. These are evident through the presence of stomatal crypts, which contain multiple stoma positioned far away from outer line of the lower epidermis. There are also trichomes present near the openings of the stomatal crypts. Both the presence of the stomatal crypts and the trichomes located inside of them point to adaptations for survival in very dry climates. The increased presence of plant fibers in the leaves allows the leaf to maintain its shape even when its other cells are plasmolyzed in dry spouts. Its multiple epidermis and thick cuticle allows the plant to handle high ultra violet radiation.
Regardless of the fact that every part of the plant is toxic to humans and other animals, it is one of the most widely grown plants in the world due to its drought-resistance with uses including ornamental, medicine, and wind-blocking. The oleander plant is also the official flower of the city of Hiroshima as it was the first plant to flower there after the destruction of the city by nuclear blast. -Chris Barrett
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I think Tuesdays are good days for cross sections. Don’t you feel that urge to perform thin slicing of plant tissue on Tuesdays? Seriously, what could be better than identifying monocots and dicots through vascular bundle arrangements!? I can’t think of any other way I’d rather spend my Tuesday. So lucky for me, this week in Oregon State plant structures lab we did exactly that! By looking at the location of the vascular bundles in plant stems, we were able to identify the plant species as monocot or dicot, assuming this wasn’t already known. Taking a close look at the vascular bundles, distinguishing of the individual components of the vascular bundles as well as the surrounding tissue was pretty clear with proper stain types and methods. Corn (Zea mays) is monocot and is in figure 1 and 2 below. TBO stain was used to provide contrast between cell types depending on the compounds present in the cell. Staining with TBO or Toluidine Blue O, will stain pectin substances pink to reddish purple, and lignin, blue or blue green. The vascular bundles are clearly stained blue which is due to the lignin of the secondary cell wall of the tracheid’s and vessel elements, the components of xylem. Phloem is the other transport tissue type found in the vascular bundles, which is composed of sieve tube elements and its dependable loving friend, the companion cells. I remember xylem and phloem and the way materials generally flow through them by saying 'xylem', in a high, squeaky voice, making me think up, and 'phloem', in a low deep voice for down. Say these words out loud a few times and hopefully it sticks in your memory for years to come like it has for myself. The xylem, transports water and minerals from the roots to the shoots and phloem carries sugars, nutrients, lipids, organics, and sad but true, viruses on a bad day. Surrounding the xylem and phloem is a sheath of sclerenchyma which helps with support and also stains thanks to its lignin found it in. Figure 1 and 2 below both help in identifying the regions referenced above.Figure 1 is a corn stem (Zea mays) cross section with TBO staining. In the image you can see the vascular bundles spread throughout the stem with parenchyma cells filling the space between the bundles and the epidermis encasing the both of them. Because the stem cross section was taken near a shoot, the tissue surrounding the epidermis of the stem is young leaf tissue. This image was taken at 40x magnification with a compound microscope. (Prepared by Lucas and photographed by Taylor) Figure 2 is a corn stem (Zea mays) cross section with TBO staining. This image is actually a more zoomed view of the same image above in figure 1. The larger cells in the vascular bundles are vessel elements. You can see two on the outside of the bundle and one or so at the bottom or top depending on orientation. The one or so at the top or bottom with the darker stained outer walls are dead and have possibly been filled with air making them look like bubbles in the slide. Vessel elements are larger transport tubes than the tracheid’s found in the xylem and are prone to air bubbles if breaks in the water tension occurs. This image was taken at 100x magnification with a compound microscope. (Prepared by Lucas and photographed by Taylor) The meristematic regions of the plant are where the new tissue is formed. There are many locations in the plant this is essentially happening. In the figures 1 and 2 above, a region running through the vascular bundles termed, procambium, creates new cells through mitosis. The procambium promotes radial growth of the plant by providing new vascular tissue to replace the non-functioning xylem and phloem. The secondary xylem and phloem get pushed away from the vascular cambium as primary vascular tissue is created giving the stem girth over time. While the pro-cambium provides lateral growth the apical meristem found at the growing tips of plants, (roots and shoots) generates upward and downward growth. Below in figure 3, you can see where the growth is taking place and the name of the region this is occurring. Figure 3 is a prepared slide of longitudinal section of a Coleus shoot meristem. Staining was used, which is apparent in the dividing cells (purple). In the image the leaf primordia, apical meristem, axillary buds, and vascular bundle are all visible. The image was taken at 40x on a compound microscope. (Pre prepared and photographed by Taylor) Figure 4 is an image under a dissecting microscope of a Coleus shoot meristem. In the shoot apical meristem cells are dividing by mitosis and forming new daughter cells that have yet to undergo differentiation. The swelling of these primary cell vacuoles will cause the shoot to move upwards causing primary growth. At some point these cells will differentiate into dermal, vascular, or ground tissue systems. (Prepared by John and photographed by Taylor) - Taylor Bates |
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