#23 - Dec 2018 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz
XYLEM December/January: Summer. When pot plants die. Last year about this time I wrote about pot-plant care when you’re away on holiday - see #3 Pot plant watering. Today, it’s about my first love – plant sap.
Sap flow to plants, is what blood flow is to animals. Essential. Ever wondered how sap flows in plants, with no heart to pump it along?
Plants have two sorts of sap - xylem sap (this issue) and phloem sap (next).
Soil: Water enters the roots from the soil by osmosis (almost pure water outside – strong sugar+minerals solution inside).
Xylem: Water flows from roots to leaves in the xylem. The xylem is a bundle of tiny tubes each about 0.2 mm diameter. The tubes are the dead remains of cylindrical cells, each about 0.5 mm long, aligned end-to-end, with their end-walls gone. The xylem tubes (vessels) have reinforced side walls and look much like miniscule vacuum cleaner hoses.
You know xylem as ‘wood’. Look at the end of a block of wood and you can just see the tiny tubes (bigger in balsa, ash and oak). The xylem lies just beneath the bark of a tree. A new 'growth ring' of xylem forms each year. The stringy bits in celery are mostly xylem too.
Air: Water escapes the leaves through tiny pores (Google stomata). Once in the air, water exists as a vapour and is measured as relative humidity (RH).
The pump: Sap flow is driven by water potential difference. Water potential (usual symbol Ψ, the uppercase Greek letter ‘psi’ - try to say 'psi', a bit tricky) measures the availability of water.
The units of Ψ are mega pascals (MPa). In a glass of water on the kitchen bench, Ψ is 0 MPa. The values of Ψ in soil, plants and air are usually negative – i.e., water is less available in these places, than in a glass of water. Soil: Here Ψ lies between 0 MPa (waterlogged) and -0.1 MPa (OK for a cactus); average soil Ψ is about -0.05 MPa.
Plant: Here Ψ lies between 0 MPa (predawn) and -2 MPa (afternoon, plants may wilt); average plant Ψ is about -1 MPa.
Air: Here Ψ lies between 0 MPa (foggy) and -100 MPa (at 50% RH); average air Ψ is about -50 MPa (70% RH).
So, water flows ‘downhill’ as it goes from the soil (Ψ slightly negative), through the plant (Ψ more negative), to the air (Ψ very negative). That’s the xylem sap pump for you…! ___________________________________
XYLEM December/January: Summer. When pot plants die. Last year about this time I wrote about pot-plant care when you’re away on holiday - see #3 Pot plant watering. Today, it’s about my first love – plant sap.
Sap flow to plants, is what blood flow is to animals. Essential. Ever wondered how sap flows in plants, with no heart to pump it along?
Plants have two sorts of sap - xylem sap (this issue) and phloem sap (next).
Soil: Water enters the roots from the soil by osmosis (almost pure water outside – strong sugar+minerals solution inside).
Xylem: Water flows from roots to leaves in the xylem. The xylem is a bundle of tiny tubes each about 0.2 mm diameter. The tubes are the dead remains of cylindrical cells, each about 0.5 mm long, aligned end-to-end, with their end-walls gone. The xylem tubes (vessels) have reinforced side walls and look much like miniscule vacuum cleaner hoses.
You know xylem as ‘wood’. Look at the end of a block of wood and you can just see the tiny tubes (bigger in balsa, ash and oak). The xylem lies just beneath the bark of a tree. A new 'growth ring' of xylem forms each year. The stringy bits in celery are mostly xylem too.
Air: Water escapes the leaves through tiny pores (Google stomata). Once in the air, water exists as a vapour and is measured as relative humidity (RH).
The pump: Sap flow is driven by water potential difference. Water potential (usual symbol Ψ, the uppercase Greek letter ‘psi’ - try to say 'psi', a bit tricky) measures the availability of water.
The units of Ψ are mega pascals (MPa). In a glass of water on the kitchen bench, Ψ is 0 MPa. The values of Ψ in soil, plants and air are usually negative – i.e., water is less available in these places, than in a glass of water. Soil: Here Ψ lies between 0 MPa (waterlogged) and -0.1 MPa (OK for a cactus); average soil Ψ is about -0.05 MPa.
Plant: Here Ψ lies between 0 MPa (predawn) and -2 MPa (afternoon, plants may wilt); average plant Ψ is about -1 MPa.
Air: Here Ψ lies between 0 MPa (foggy) and -100 MPa (at 50% RH); average air Ψ is about -50 MPa (70% RH).
So, water flows ‘downhill’ as it goes from the soil (Ψ slightly negative), through the plant (Ψ more negative), to the air (Ψ very negative). That’s the xylem sap pump for you…! ___________________________________