#24 - Feb 2019 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz
PHLOEM February: Late-summer. Plants have two sorts of sap - xylem sap and phloem sap. Last issue (#23 Xylem) I described how water is transported from the soil solution, into the roots, along bundles of tiny tubes (the xylem), to the leaves and then to the air through tiny adjustable pores (stomata). This issue I promised you the phloem sap story.
Phloem transport: Leaves are sugar factories (photosynthesis). They use carbon dioxide (CO2), water (H2O) and sunlight energy to make sugar (CH2O) and oxygen (O2). The growing roots, shoots and fruits use this sugar as food. But the sugar must be transported from the leaves (sources) to the distant roots, shoots and fruits (sinks) where it’s consumed. The sugar is transported in the phloem.
Phloem: In a tree, the (soft, white) phloem lies just under the (crusty, brown) bark and just outside the (hard) wood. The phloem is made up of bundles of many tiny tubes (sieve tubes). Each sieve tube is made up of long rows of cylindrical cells (sieve elements). The sieve elements are aligned end to end and interconnect through their adjoining end walls by very tiny holes (the round, end walls look like sieves). Each sieve element is about 0.2 mm long and 0.02 mm in diameter. Unlike the dead cells of the xylem, the sieve elements are alive.
Phloem sap: The phloem sap is a strong sugary solution (about 13 teaspoons of sugar in a teacup). The strong sugar draws water into the sieve elements (osmosis) inflating them to high pressure - about 10-times the pressure in a car tyre. The stronger the sugar solution, the higher the pressure. But what makes this viscous sugar solution flow along the tiny sieve tubes?
Phloem pump: Sugar is loaded into the sieve elements in the source leaves (high concentration = high pressure) and unloaded from the sieve elements in the sink organs - shoots, roots, fruits (lower concentration = lower pressure). The source-to-sink pressure difference makes the sap flow from the high-pressure source end to the lower-pressure sink end. Just as water flows in a garden hose from the high-pressure tap end to the lower-pressure nozzle end.
Tricks: Plants use all sorts of clever tricks to boost sugar flow to the most important sinks (e.g. fruits) and to starve the less important sinks (e.g. roots). That’s why roots stop growing in summer when fruits are growing.___________________________________
PHLOEM February: Late-summer. Plants have two sorts of sap - xylem sap and phloem sap. Last issue (#23 Xylem) I described how water is transported from the soil solution, into the roots, along bundles of tiny tubes (the xylem), to the leaves and then to the air through tiny adjustable pores (stomata). This issue I promised you the phloem sap story.
Phloem transport: Leaves are sugar factories (photosynthesis). They use carbon dioxide (CO2), water (H2O) and sunlight energy to make sugar (CH2O) and oxygen (O2). The growing roots, shoots and fruits use this sugar as food. But the sugar must be transported from the leaves (sources) to the distant roots, shoots and fruits (sinks) where it’s consumed. The sugar is transported in the phloem.
Phloem: In a tree, the (soft, white) phloem lies just under the (crusty, brown) bark and just outside the (hard) wood. The phloem is made up of bundles of many tiny tubes (sieve tubes). Each sieve tube is made up of long rows of cylindrical cells (sieve elements). The sieve elements are aligned end to end and interconnect through their adjoining end walls by very tiny holes (the round, end walls look like sieves). Each sieve element is about 0.2 mm long and 0.02 mm in diameter. Unlike the dead cells of the xylem, the sieve elements are alive.
Phloem sap: The phloem sap is a strong sugary solution (about 13 teaspoons of sugar in a teacup). The strong sugar draws water into the sieve elements (osmosis) inflating them to high pressure - about 10-times the pressure in a car tyre. The stronger the sugar solution, the higher the pressure. But what makes this viscous sugar solution flow along the tiny sieve tubes?
Phloem pump: Sugar is loaded into the sieve elements in the source leaves (high concentration = high pressure) and unloaded from the sieve elements in the sink organs - shoots, roots, fruits (lower concentration = lower pressure). The source-to-sink pressure difference makes the sap flow from the high-pressure source end to the lower-pressure sink end. Just as water flows in a garden hose from the high-pressure tap end to the lower-pressure nozzle end.
Tricks: Plants use all sorts of clever tricks to boost sugar flow to the most important sinks (e.g. fruits) and to starve the less important sinks (e.g. roots). That’s why roots stop growing in summer when fruits are growing.___________________________________