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Interesting puzzle, perhaps not so simple. Any cell system like a live lea=
f is bound to be somewhat denser than pond water, regardless of denser star=
ch, so will naturally sink at all times unless kept buoyant by the extracel=
lular gas bubbles or some other modification. What seems to need explainin=
g is why the bubbles are maintained in summer (dissolved gases can pass thr=
ough lipid cell membranes quickly, so you=92d expect them to dissolve out i=
nto the surrounding pond), and why they disappear in winter. He doesn=92t =
seem to know if it=92s CO2 or O2 in the bubbles, or both. Perhaps they are=
largely O2 in summer and dissolve and normally pass out of the leaf to oxy=
genate the pond and dissolved CO2 moves in, but leaf photosynthesis is suff=
iciently high in summer to maintain them, despite these losses. In winter =
gas production simply stops, and the non-buoyant plants sink. As winter en=
ds, respiration produces bubbles with a different gas, CO2, and they rise a=
gain then switch over to O2 as photosynthesis picks up.
It therefore seems likely that these leaves have some special surface coati=
ng that slows down gas exchange with the pond. Maybe they still retain sto=
mata under the leaf which normally facilitate gas exchange for leaves in ai=
r, but these are modified to block or regulate gas exchange in water? Mayb=
e this is already known, just not to us here?
Steve
On Mar 10, 2018, at 10:45 AM, David <dwebster@glinx.com<mailto:dwebster@gli=
nx.com>> wrote:
Hi Nancy & All,
I admit not having wondered about this but I think he makes a simple pr=
ocess complicated. The air pockets to which he refers are presumably interc=
ellular spaces which in the absence of photosynthesis presumably can become=
water filled. Starch has a specific gravity of 1.5 g/mL and, in cooler wea=
ther starch will accumulate when respiration slows more than photosynthesis=
and the submarine will sink. With warming, growth resumes, respiration and=
starch consumption rates increase and the submarine rises.
Yt, DW, Kentville
------ Original Message ------
From: "nancy dowd" <nancypdowd@gmail.com<mailto:nancypdowd@gmail.com>>
To: naturens@chebucto.ns.ca<mailto:naturens@chebucto.ns.ca>
Sent: 3/10/2018 9:08:02 AM
Subject: [NatureNS] Duckweed
This is an interesting article on Duckweed seasonal disappearance and reapp=
earance in the Spring 2018 issue of Northern Woodlands Magazine.https://nor=
thernwoodlands.org/outside_story/article/duckweed-migration
I had never really thought about where it went in Fall or how it re-emerges=
in Spring. Such an important floating pond plant in productive fresh water
Nancy D
Sent from my iPad
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<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3DWindows-1=
252">
</head>
<body style=3D"word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-lin=
e-break: after-white-space;">
Interesting puzzle, perhaps not so simple. Any cell system like a liv=
e leaf is bound to be somewhat denser than pond water, regardless of denser=
starch, so will naturally sink at all times unless kept buoyant by the ext=
racellular gas bubbles or some other
modification. What seems to need explaining is why the bubbles are m=
aintained in summer (dissolved gases can pass through lipid cell membranes =
quickly, so you=92d expect them to dissolve out into the surrounding pond),=
and why they disappear in winter. He
doesn=92t seem to know if it=92s CO2 or O2 in the bubbles, or both. =
Perhaps they are largely O2 in summer and dissolve and normally pass out of=
the leaf to oxygenate the pond and dissolved CO2 moves in, but leaf photos=
ynthesis is sufficiently high in summer to
maintain them, despite these losses. In winter gas production simply=
stops, and the non-buoyant plants sink. As winter ends, respiration =
produces bubbles with a different gas, CO2, and they rise again then switch=
over to O2 as photosynthesis picks up.
<div><br>
</div>
<div>It therefore seems likely that these leaves have some special surface =
coating that slows down gas exchange with the pond. Maybe they still =
retain stomata under the leaf which normally facilitate gas exchange for le=
aves in air, but these are modified to
block or regulate gas exchange in water? Maybe this is already known=
, just not to us here?</div>
<div>Steve <br>
<br>
<div>
<div>On Mar 10, 2018, at 10:45 AM, David <<a href=3D"mailto:dwebster@gli=
nx.com">dwebster@glinx.com</a>> wrote:</div>
<br class=3D"Apple-interchange-newline">
<blockquote type=3D"cite">
<div style=3D"font-family: Tahoma; font-size: 12pt; font-style: normal; fon=
t-variant: normal; font-weight: normal; letter-spacing: normal; line-height=
: normal; orphans: auto; text-align: start; text-indent: 0px; text-transfor=
m: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text=
-stroke-width: 0px;">
<div>Hi Nancy & All,</div>
<div> I admit not having wondered about this but I think he ma=
kes a simple process complicated. The air pockets to which he refers are pr=
esumably intercellular spaces which in the absence of photosynthesis presum=
ably can become water filled. Starch has a
specific gravity of 1.5 g/mL and, in cooler weather starch will accumulate=
when respiration slows more than photosynthesis and the submarine will sin=
k. With warming, growth resumes, respiration and starch consumption rates i=
ncrease and the submarine rises.</div>
<div>Yt, DW, Kentville</div>
<div><br>
</div>
<div>------ Original Message ------</div>
<div>From: "nancy dowd" <<a href=3D"mailto:nancypdowd@gmail.co=
m">nancypdowd@gmail.com</a>></div>
<div>To:<span class=3D"Apple-converted-space"> </span><a href=3D"mailt=
o:naturens@chebucto.ns.ca">naturens@chebucto.ns.ca&