ULTRASTRUCTURE OF VEGETATIVE AND DIVIDING CELLS OF THE UNICELLULAR RED ALGAERHODELLA VIOLACEAANDRHODELLA MACULATA1

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Abstract
Rhodella violacea(Kornmann) Wehrmeyer andRhodella maculataEvans were investigated for ultrastructural details of vegetative and dividing cells.Rhodella violaceahas a nuclear projection into the pyrenoid similar to that found inR. maculata, although the nuclear projection inR. maculatatraverses a starch‐lined area before contacting the pyrenoid. Unlike most, red algae, the twoRhodellaspecies lack a peripheral encircling thylakoid in the chloroplast and have dictyosomes associated solely with endoplasmic reticulum (ER) instead of with both mitochondria and ER. Both species also have a well‐developed peripheral system of ER connected to the plasmalemma by tubules, a situation found only in red algal unicells, Cell division was studied primarily inR. violacea;a less thorough examinationof R. maculatashowed no essential differences. Both have small, double‐ringed, nucleus‐associated organ files (NAOs) surrounded by moderately electron‐dense material, metaphase–anaphase polar gaps in the nuclear envelope, absence of perinuclear ER. and short interzonal spindles. This pattern of mitosis is similar in most respects to that reported in the unicellFlintiella.Following mitosis, microtubules extend from the region of each NAO to its associated nucleus and to the undivided pyrenoid. The NAOs appear to apply tension to the nuclei and the pyrenoid and may be the mechanism for ensuring equal partitioning of both organdies. Two different forms of pyrenoid‐nucleus association occur during mitosis. Nuclear projections into the pyrenoid, prevalent during interphase and early stages of mitosis, recede at metaphase. Then, the pyrenoid extends protrusions into the nuclear polar areas, forming a cup that partially surrounds the nucleus. Cell division and vegetative characters confirm the close taxonomic affinity of these two species ofRhodellaand support their separation from the genusPorphyridium.