[neilep (OP)]

What is the liquidy sap (that looks like water but is not dew) that some plants exude ?

......what is i and why do they do it..and how ?

[Karen W.]

I always thought of it as glucose like what the clear fluid we exude or blood for us. But plant glucose! LOL I really don't know though!

[Karen W.]

It could be plant sap also maybe produced by injuries to the plants!

[Andrew K Fletcher]

What is the liquidy sap (that looks like water but is not dew) that some plants exude ?

......what is it and why do they do it..and how ?


Ok let’s have a stab at this for you Neil as it poses a considerable amount of problems for the existing literature and is a valid question that requires a response.

Circulation under favourable weather conditions effectively releases salts and sugars down the plant or grass, which will inevitably cause the fluids inside the plant to rotate. It does this because the tiny pulses of salts and sugars released cause a positive pressure in front of the falling sap and a negative tension behind the falling sap. Picture the salts as a liquid plunger in a syringe that is withdrawn under water, here you see a pushing force behind the plunger forcing water back towards the end of the plunger you are pulling on, but at the same time water is drawn in through the open end / business end of the syringe body.

In the morning when the air is saturated or close to dew point, the circulation caused by evaporation is shut down, well not entirely shut down to be precise. On the contrary, the salts still flow down and there is still a return flow but the circulation of the fluids changes in the plant. Now we see a predominant downward pressure occurring. Picture a U tube filled with water and then add salt solution to one side. This causes the salt free side to exude from the top of the salt free side while at the same time the water level in the salt laden side goes down respectively. I suspect that cavitations under the stress of compromised evaporation takes place during high humidity and rather than the complete circulation under normal conditions there is a release of tension in the return flow followed by an equal increase in positive pressure in the downward flow side.

This same scenario occurs in the Autumn in deciduous trees as the leaves fall off again compromised by high humidity and higher rainfall which inevitably dilutes the mineral in the soil-water reducing the density of the sap in the xylem which would counteract the density changes at the leaf in a double whammy. There is a shift of salts and minerals to the roots during the fall, and an increase in positive pressure in the roots making this time ideal for planting trees as roots are forced down through the soil and grow more rapidly during this time than any other time of the year.

In text books we see reference to root pressure as sap exudes from a cut stem. Cutting the stem also compromises the circulation because you have removed the leaves that water evaporates from. These experiments show a tiny drop of water forming on the top of the cut stem and the reference to the roots pushing the fluid out must refer to the fluid tension inside the root and stem being released much the same as the explanation given above.   
  

[Karen W.]

Nice explanation Andrew!

[neilep (OP)]

FANTASTIC EXPLANATION !!

I am so sorry it took me a while to thank Mr Andrew for his awesome explanation.

I always really appreciate the time and effort that people put in to answer questions on this site.

THANK YOU ANDREW !

[Andrew K Fletcher]

The Experiment at Brixham Overgang Cliffs where water flowed vertical up a single 6 mm bore tubing using 10 mils of salt solution, demonstrating that a tiny amount of denser solution can lift effortlessly many thousands of times it’s own volume in water without any artificial aids, demonstrating clearly a non living physical cause of bulk flow in plants trees, animals and humans. The 10 metre limit for lifting water clearly needs some serious revision.

Online experiment details
http://www.the-tree.org.uk/TreeTalk/3Spring2003/Gravity/gravity1.htm

http://www.myspace.com/inclined_bed_therapy

[Andrew K Fletcher]

In this short video there is an example of water exuding from an open ended U tube showing precisely how water exudes from a cut stem. When viewing the part about spirit levels and while observing the level differences between the tube, picture a pair of scissors cutting through both tubes as would be the case in a tree or plant. What would happen to the water levels?

Andrew