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>>>> --_000_29C78FB668C5425F9729ABA4EEDC204Ddalca_ Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Hi Dave, I=92d forgotten about this, but isn=92t the primary observation to explain = that the plants routinely sink in the winter and rise again in the spring? = What the mechanism is, is an interesting but secondary consideration. In= your =91experimental=92 case with lighted duckweed, it should be producing= and accumulating some sort of gas mixture by September's photosynthesis, m= aybe plus some concurrent respiration. Your =91control', dark duckweed, sh= ould have no photosythesic activity, so only respiration should produce any= gas at all, presumably CO2. The problem is that neither =91expt' nor =91control' actually sank on Sept = 27. Since the plant material including any dense starch should anyway have= an average density >1g/cc (i.e. greater than plain water*), excluding any = gas spaces, both cases must still be floating according to some undissolved= gas inclusions, though not necessarily the same mix of CO2/O2 in the two c= ases. You get an A+ for industry and design, but Lemna gets an F for coope= ration. *Something I=92d not considered that you may know about but I don=92t is oi= l/fat production in plants, which obviously is important for some land plan= ts (canola, sesame, olives etc). Oils have specific gravities <1, around 0= .9 g/cc, so enough oil volume could be sufficient to float a water plant wi= thout invoking gas spaces. If something dense like starch gets converted t= o an oil in spring, could that contribute, or is this just fantasy? Is it possible that the trigger for sinking, whatever its mechanism, is a p= articular drop in day length that had not yet been closely approached by Se= pt 27, and especially in your toasty living room (temperature might be anot= her factor)? If you haven=92t flushed them in frustration, you should con= vey both your test subjects to an unheated outhouse and extend your experim= ent there at least to freeze-up. Steve --------------------------------------------------------- On Sep 27, 2018, at 12:35 PM, David <dwebster@glinx.com<mailto:dwebster@gli= nx.com>> wrote: Dear All, I have had my eye open this summer for some Lemna to test the gas compo= sition idea; the more soluble CO2 predominating as days shorten leading to = flooding of gas spaces and sinking; photosynthesis predominating in spring = as days lengthen leading to flooded air spaces being filled by less soluble= O2 and rise to the surface. And I was able to collect some on Sept 21 without having to wade in muc= k of uncertain depth. So I set up a simple trial; some kept in darkness, ex= cept for a few minutes daily for observation, and some kept on a windowsill= . By Sept 27 those kept in darkness showed no tendency to sink but were n= oticeably less green than those exposed to daylight and some artificial lig= ht in the evening. This leads to the conclusion that flooding of intercellu= lar gas spaces when CO2 predominates does not cause sinking of Lemna. The = converse explanation that Lemna rises to the surface in the spring when int= ercellular spaces become filled by less soluble O2 is accordingly voided. The explanation given in the link posted by Nancy on Oct 3, 2017 is con= sequently more sound. Starch accumulation in the fall. as photosynthesis ex= ceeds respiration at lowering temperature, leads to sinking. Starch consump= tion, as temperature warm in the spring and new buds start to grow, leads t= o rise to the water surface. [density of starch is high; 1.5 g/cm^3] Yt, DW, Kentville ------ Original Message ------ From: "Stephen Shaw" <srshaw@Dal.Ca<mailto:srshaw@Dal.Ca>> To: "naturens@chebucto.ns.ca<mailto:naturens@chebucto.ns.ca>" <naturens@che= bucto.ns.ca<mailto:naturens@chebucto.ns.ca>> Sent: 3/10/2018 3:20:47 PM Subject: Re: [NatureNS] Duckweed 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 impo