Sieve-Element Ontogeny in the Aerial Shoot of Equisetum hyemale L.

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Abstract
The aerial shoots of Equisetum hyemale L. var. affine (Engelm.) A. A. Eat. were examined with the electron microscope as part of a continuing study of sieveelement development in the lower vascular plants. Young E. hyemale sieve elements are distinguishable from all other cell types within the vascular system by the presence of refractive spherules, proteinaceous bodies which develop within dilated portions of the endoplasmic reticulum (ER). Details of cell wall thickening differ between protophloem and metaphloem sieve elements. Following cell wall thickening the ER increases in quantity and aggregates into stacks. Shortly thereafter, nuclear degeneration is initiated. During the period of nuclear degeneration some cytoplasmic components-dictyosomes, microtubules and ribosomes-degenerate and disappear, while organelles such as mitochondria and plastids persist. The latter undergo structural modifications and become parietal in distribution. Eventually the massive quantities of ER are reduced, leaving the lumen of the cell clear in appearance. At maturity the plasmalemma-lined sieve element contains a parietal network of tubular ER, as well as mitochondria, plastids, and refractive sphemh At this time many of the spherules are discharged into the region of the wall. Sieveelement pores occur in both lateral and end walls. At maturity many pores are traversed by large numbers of ER membranes. The metaphloem sieve elements of the mid-internodal regions apparently are sieve-tube members. The connections between mature protophloem sieve elements and pericycle cells are associated with massive wall thickenings on the pericyclecell side.

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