Investigating transport in plants

Ringing experiments:

Woody stems have an outer protective layer of bark on the inside of which is a layer of phloem that extends all around the stem. Inside the phloem layer is xylem

• At the start of a ringing experiment, a section of the outer layers (protective layer and phloem) is removed around the complete circumference of a woody stem while it is still attached to the rest of the plant
• After a period of time, the region of the stem immediately above the missing ring of tissue is seen to swell 
• Samples of the liquid that has accumulated in this swollen region are found to be rich in sugars and other dissolved organic substances
• Some non-photosynthetic tissues in the region below the ring (towards the roots) are found to wither and die, while those above the ring continue to grow

These observations suggest that removing the phloem around the stem has led to -

• the sugars of the phloem accumulating above the ring, leading to swelling in this region
• the interruption of the flow of sugars to the region below the ring and the death of tissues in this region

The conclusion drawn from this type of ringing experiment is that phloem, rather than xylem, is the tissue responsible for translocating sugars in plants.

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