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Index of Subjects --_000_F3FE0AE5B9514A4A95754CE98894C7CAdalca_ Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable 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 --_000_F3FE0AE5B9514A4A95754CE98894C7CAdalca_ Content-Type: text/html; charset="Windows-1252" Content-ID: <4AE9F68B7AA2494DB2135104281B2FEC@CANPRD01.PROD.OUTLOOK.COM> Content-Transfer-Encoding: quoted-printable <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&