# Phun Physiology Phact #3



## Phil Edwards (Jan 22, 2004)

Plants have two methods of moving solutions through their bodies. 

Xylem= Large, hardened cells that transport water and dissolved minerals from the roots to the shoots. 

Phloem= Transports photosynthate saturated sap from the mature leaves to the non-photosynthetic organs of the plant.


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## plantbrain (Jan 23, 2004)

Name the types of Phloem and Xylem cells............

Regards, 
Tom Barr


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## Phil Edwards (Jan 22, 2004)

LOL Tom, you crack me up.

Xylem: Vessel Elements and Tracheids 
Phloem: Sieve Tube Element, Companion Cells, or Sieve Cells in Gymnosperms.


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## dennis (Mar 1, 2004)

And how do they accomplish these feats? Certainly magic is not involved!


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## Phil Edwards (Jan 22, 2004)

That's a great question Dennis, and I can only answer half of it for sure. It's actually a very interesting and complex topic which is better served as a long discussion, but I'll try to highlight the best parts here. 

I know how plants move water through their Xylem system in terrestrial culture, but not when submerged. My best current guess is that they manipulate the ion concentration within their cells, particularly those of their roots, aerial or substrate, to cause water to enter via an osmotic pathway.

In their sap system, Phloem, plants actively transport photosynthates into the upper Phloem cells which causes H2O to enter the cell osmotically. That H2O causes a pressure differential which moves the sap to areas of lower pressure. That low pressure is created by the plant sucking up the photosynthates causing the H2O in the sap to diffuse elsewhere. As long as there are areas of the plant that are removing the photosynthates from the sap and the leaves are loading photosynthates into the sap there will be a continuous pressure gradient which will move sap to the places that need it. 

The Xylem H2O system generally moves only upwards (again I've only seen details of this for terrestrial plants where evaporation is a key element), but Phloem moves sap both upwards and downwards depending on the strength of the sap sinks. 

Regards,
Phil


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