Plant cell

[17] Parenchyma cells have thin, permeable primary walls enabling the transport of small molecules between them, and their cytoplasm is responsible for a wide range of biochemical functions such as nectar secretion, or the manufacture of secondary products that discourage herbivory.

The role of this cell type is to support the plant in axes still growing in length, and to confer flexibility and tensile strength on tissues.

Consequently, sclereids and fibres are typically dead at functional maturity, and the cytoplasm is missing, leaving an empty central cavity.

Sclereids form the hard pit wall of peaches and many other fruits, providing physical protection to the developing kernel.

Fibres are elongated cells with lignified secondary walls that provide load-bearing support and tensile strength to the leaves and stems of herbaceous plants.

Xylem is a complex vascular tissue composed of water-conducting tracheids or vessel elements, together with fibres and parenchyma cells.

Vessel elements are hollow xylem cells without end walls that are aligned end-to-end so as to form long continuous tubes.

The sieve tube elements lack nuclei and ribosomes, and their metabolism and functions are regulated by the adjacent nucleate companion cells.

The plant epidermis is specialised tissue, composed of parenchyma cells, that covers the external surfaces of leaves, stems and roots.

The epidermis of all aerial organs, but not roots, is covered with a cuticle made of polyester cutin or polymer cutan (or both), with a superficial layer of epicuticular waxes.