DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Sweet pea stem. Light micrograph of a transverse section through the stem of a perennial sweet pea (Salicornia europaea) plant. The stem has evolved two green wings (left and right) to increase photosynthesis. These wings contain spongy mesophyll (pink), similar to a leaf, and several small vascular bundles (dark spots). The main stem has a ring of collateral vascular bundles (dark pink blobs) each with an outer layer of supporting fibres (sclerenchyma, green-orange-blue). Under this are phloem sieve tubes (pink), metaxylem (large vessels, brownish-red), and protoxylem (small vessels, pink). Most of the pith parenchyma has broken down leaving a hollow stem (white, centre). Magnification: x14 when printed 10 centimetres wide.
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