Floëem

Floëem is the phloem, the vascular tissue in many plants responsible for transporting organic nutrients, especially sugars such as sucrose, from photosynthetic organs to non-photosynthetic tissues. In Dutch botany, floëem is the term used for this tissue, which forms part of the plant’s transport system alongside the xylem.

The floëem is composed mainly of sieve elements, which in angiosperms are sieve tube elements arranged end to end to form sieve tubes. These are usually accompanied by companion cells that regulate transport and provide metabolic support. Other components of floëem include phloem parenchyma, which stores and distributes solutes, and phloem fibers that provide structural strength. In gymnosperms, the conducting cells are often sieve cells rather than sieve tubes, and companion cells may be absent or replaced by different supportive cells.

Functionally, floëem moves photosynthates and other solutes from source tissues, such as mature leaves, to sinks, including developing leaves, roots, fruits, and seeds. The prevailing mechanism is the pressure-flow hypothesis, which posits that loading solutes into the floëem at the source creates an osmotic gradient that draws water into the tissue, generating turgor pressure that pushes sap toward sinks where solutes are unloaded. This process is energy-dependent and involves active transport at loading and unloading sites, with solutes moving through sieve plates between sieve elements.

Floëem development arises from the plant’s vascular tissue, and its exact structure varies among plant groups. Proper floëem function is essential for growth, reproduction, and storage, and disruptions can impact crop yields and plant health.