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1. Is the wall of the xylem lignified and completely impermeable to water? If so how come the pressure flow theory (which deals with translocation of materials in phloem) states that when the photosynthetic source is loaded with sugar, water moves in from the xylem by osmosis? By what means can water cross the lignified boundaries of the xylem wall to come into the phloem then?
2. What are tracheids in xylem? Is it true that they are like vessel elements because they also connect to one another to form the xylem vessel; tracheids however connect to the other tracheids at their ends through pits, while vessel elements have their end walls completely broken down, is that true? Can tracheids and vessel elements exist in the same plant?
Thanks a lot!
A little late, but...
1) The walls of xylem are covered in pits which allow water to be diffused into and out of the xylem. Water movement is passive; water is pulled through the plant because of transpiration and capillary action. As water exits the leaves, it creates pressure in the xylem that causes water to move up the plant. Just as well, water has a natural tendency to move to areas with less concentrated water. This is known as water potential. When minerals enter a plant's roots, the water potential in the roots decreases because the overall water content of the roots decrease. Water naturally enters the roots and then carries the nutrients through the plant. It does the same thing with sugar. Water enters the plant's sieve-tubes and transports the sugar throughout the plant body . The tracheids, vessel elements, and sieve-cells are all lines with pits and have perforations and open discs for transport. Xylem does have lignified cell walls, but they're covered in the pits for diffusion.
2) You have it pretty spot-on with tracheids. They're long vascular cells that are stacked on top of each other, unlike vessel elements which are, for the most part, opened at the ends. However, both tracheids and vessel elements CAN exist in the same plant. You won't find vessel elements in conifers and lower plants, but you'll find both tracheids and vessel elements in the angiosperms, flowering plants. But, again, like most of biology, things are pretty fluid; vessel element-like cells have been found in lower plants alongside tracheids. Cells like vessel elements have evolved multiple times independently, but they in no way replace tracheids in plants that have them. Tracheids are much smaller and more numerous even in angiosperms.
3 posts • Page 1 of 1
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