DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY
Vascular bundle in a celery stalk (Apium graveolens), scanning electron micrograph (SEM). Celery stalks have numerous vascular bundles which act as a transport system. Shown here is a single vascular bundle in a celery stalk. The vascular bundle contains two types of conductive tissue: phloem (top area) and xylem (lower area). Phloem is tissue that transports larger, organic molecules through the plant. It is composed of several cell types, such as: sclerenchyma, parenchyma, sieve elements and companion cells. The sieve element and companion cell are found closely associated with each other in sieve element-companion cell complex. Xylem is tissue that is responsible for transporting water and nutrients through the plant. Xylem consists of a variety of specialized, water-conducting cells known as tracheary elements (tracheids). Surrounding the vascular bundle are supporting plant collenchyma cells. Magnification: x30 when shortest axis printed at 25 millimetres.
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