Observations on root and stem contraction in cycads (Cycadales) with special reference to Zamia pumila L.

Cycads are pachycaulous plants possessing either aerial and columnar (arborescent) stems or subterranean and tuberous fleshy stems. In some taxa, such as Zamia pumila and Stangeria eriopus , the vertical shoots are subterranean; the shoot apex usually remains underground even though new shoot increments arc added as a result of leaf development. The external surface of these stems has a winkled appearance in the distal regions and the height of the leaf scars decreases basipetally This is because both the fleshy tap root and the stem contract, pulling the shoot underground. This contraction results from the collapse of horizontal rows of cells in both the pith and cortex. The vascular cylinder which appears straight in longitudinal sections becomes sinuously distorted as this collapse of cells occurs.