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NZ Plants

The stem

Fern stems conduct nutrients and water and support large fronds. To do this there is a well developed vascular tissue system. Many of their cell walls are also strengthened with deposits of a compound called lignin.

Creeping stem (rhizome)

Most ferns have stems that grow horizontally on the surface of soil, rocks or tree trunks that form leaves at intervals.
Stems with these characteristics are called rhizomes.

Stem (rhizome) with young frond
photo, L Jensen

Hypolepis dicksonoides - The long, creeping rhizome found in most ferns aid in their rapid and proliferation.

More on this group of ferns 

Erect tree-like stems

Most trees support themselves by forming large deposits of wood but this tissue is absent in tree ferns.
The tall trunk of tree ferns is instead strengthened by:
--deposits of lignin which add great strength to the cell walls
--reinforcing the lower trunk with thick, interlocking mats of tiny roots

(photo, J Braggins)

Cyathea medullaris, mamaku, black tree fern -Trunks can reach 20 m and have hexagonal leaf scars. A fast growing species favouring moist, protected areas. (photo, J Braggins)

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Base of trunk, localised formation of a sheath of roots increases the girth
(photo, L Jensen)

Cyathea dealbata, silver tree fern - The increase in girth at the base of the tree fern trunk is achieved by annual accumulation of fine interlocking roots. Growth of roots is accelerated on the lower side of leaning trunks to produce buttresses. 

Lower region of trunk, fine roots form a tough sheath adding support and may join together adjacent plants.
(photo, L Jensen)

Dicksonia squarrosa, wheki - It is not unusual for adjacent trunks to be joined together by growth of their roots.

More on this group of ferns 

Base of trunk, growth of roots occurrs in annual cycles
(photo, L Jensen)

Cyathea dealbata, silver tree fern - The compacted arrangement of the growing roots and their rapid hardening of tissues creates a firm union with each other and the trunk.