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Index of Subjects Yes, Arthur and with reference to leaves/branches that appear to be otherwise free of damage. Lots of leaves & branches shed all over town and branches damaged but retained. A dandy nearby; est. 12" diameter Salix fragilis branch broken off entirely and hung upside down on a dead snag; ~60' up. Dave ----- Original Message ----- From: "Rick Whitman" <dendroica.caerulescens@gmail.com> To: <naturens@chebucto.ns.ca> Sent: Tuesday, July 08, 2014 5:23 PM Subject: Re: [NatureNS] xylem damage > Dave, > Are you referring to damage from H. Arthur ? I feel the damage is obvious > here. There was a massive loss of leaves, so many as to change the visual > impression looking at light thru the trees. Also, the cleanup is > unprecedented in 35 yrs. Branches not ripped off are weakened & not > returned to position. The most exposed leaves certainly look wilted. > > Rick Whitman. > >> On Jul 8, 2014, at 10:58 AM, David & Alison Webster <dwebster@glinx.com> >> wrote: >> >> >> Dear All, July 8, 2014 >> I have not been into woods yet but I have noticed in the yard that many >> leaves of Ash, Sweet Cherry Seedling, Sugar Maple and Norway Maple are >> somewhat wilted, especially near branch tips, even early in the morning. >> This suggests structural damage to xylem elements such that gas (water >> vapor or air) blocks water movement. >> >> One great mystery in Plant Physiology is how occlusion of xylem >> elements by vapor doesn't happen routinely which would limit plant height >> to 30' and plant growth to very humid environments. The vapor pressure of >> water at 10o C is about 9 mm Hg, 4 mm at 0o C (standard atmosphere being >> 760 mm) so at tensions greater than 751 mm Hg (at 10o C) one would expect >> the water in xylem elements to boil. And water tensions can exceed 80 atm >> (drawing on memory) with often no Ill effects. >> >> As noted in a private Dec 2013 e-mail (below) I think the explanation >> may lie in the state of water in xylem elements; not truly free water but >> bound in a gel. Analogous to the movement of electrons in a conductor, >> only a small fraction of the water in a xylem element is free to move at >> any instant. Consequent the activity of water is very low and the vapor >> pressures determined on free water do not apply. This is speculation but >> it is the only mechanism, known to me, that makes any sense. >> >> START OF PASTE\\\\\\\\\\\\\ >> Interesting that Driwater is 2% Cellulose, 98% water and is solid. It >> would not be possible to directly demonstrate the presence of a weak gel >> in Xylem elements but it would be possible to compare maximum tensions >> attained in fine glass capillaries containing various dilutions of >> cellulose gel or silica gel or various hydrogels. The presence of >> cellulose chains that are attached to the wall only at one end in xylem >> elements is not so far fetched. Assuming secondary thickening from the >> primary wall inward with cellulose units sewed into the ends of various >> strands by metabolic action, how would the end of a spiral chain ever >> manage to become attached to the wall ? When the cell contents die and >> become digested by autolysis (sp ?) are there not always going to be a >> swirl of loose cellulose ends ? >> END OF PASTE\\\\\\\\\\\\\\\\ >> >> Yt, DW >> > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2014.0.4716 / Virus Database: 3986/7812 - Release Date: 07/07/14 >
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