[NatureNS] xylem damage

From: David & Alison Webster <dwebster@glinx.com>
To: naturens@chebucto.ns.ca
References: <2BD977FDA20E4FCBB2BCD0E90715B67A@D58WQPH1>
Date: Tue, 08 Jul 2014 19:05:48 -0300
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Return-Path: <naturens-mml-owner@chebucto.ns.ca>
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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
>>
>
>
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