I. SYNONYMS: praesinophytes,

II. NUMBER: >180 species.

III. PHYLUM CHARACTERISTICS:

• A. Structure and Physiology
  • Cell Form: Generally, unicells; some (Prasinocladus) are benthic filaments.
  • Flagella: 1-8 flagella, laterally or apically inserted with sparse hairs or scales and anteriorly directed (text with tooltip) An anteriorly-directed flagellum extends in the direction of the motion of the motile cell. The interpretation is that the flagellum functions by pulling the cell. .
  • Basal Bodies: Basal bodies with roots that are cross-shaped to unilateral; those with the broad roots have a multilayered structure (text with tooltip) A multilayered structure (MLS) is a microtubular array that is at the base of the basal bodies in the motile cells of certain green algae and embryophytes. The MLS seems to be part of the microtubular organizing center for the cytoskeleton of the cell. (MLS) some with rhizoplast (text with tooltip) A rhizoplast is a striated root system that extends from the basal body tinto the cell, usually to the nuclear envelope. and broad flagellar roots (text with tooltip) Flagellar roots are microtubular structures that arise from the basal bodies and elaborate to the inside of the cell. They may connect to the nucleus (as part of a karyomastigont) or course their way into the cell such that they form other structures like axostyles. , others with flagellar roots associated with the cell surface.
  • Cell Covering: Usually covered with elaborate scales of cellulose. Some naked.
  • Chloroplasts: Variable; grass green with chlorophylls a and b, B-carotene and various xanthophylls. Eyespot (text with tooltip) An eyespot is a light-sensitive structure that does not form an image. This can be part of an organelle as in the chloroplast of certain microbial eukaryotes. It can be an elaborate structure that involves a light-sensitive swelling at the base of a flagellum (as in the euglenoids) or it can be a multicellular structure as in planarians. , when present, is always enclosed within the chloroplast.
  • Food Reserves: True starch that is deposited within the chloroplast in association with a pyrenoid.
  • Mitochondria: Plate-like cristae (text with tooltip) Flat cristae (crista, sing.) are wall-like cristae within certain mitochondria. .
  • Golgi: Present.
  • Nucleus: Presumed haploid in vegetative forms.
  • Centrioles: Not present
  • Inclusions and Ejectile Organelles: Not present.

• B. Mitosis, Meiosis and Life History
  • Mitosis: Variable, open to closed (text with tooltip) Mitosis is closed when the segregation of daughter chromosomes occurs within the bounds of the nuclear membrane (the nuclear membrane does not break down). ; with or without a persistent phycoplast (text with tooltip) A phycoplast is a microtubular array that follows mitosis and is oriented perpendicularly to the axis of the spindle. It serves to keep the daughter nuclei separated during cytokinesis. The phycoplast also serves as the organizing region in which furrowing and cell wall formation occurs after mitosis. Contrast this with a phragmoplast. ; cytokinesis by furrowing (text with tooltip) Furrowing is the means of cytokinesis in which the cell membrane pinches in from the outside to the center. Note that this is the opposite of cell plate formation in phragmoplastic plants. to cell plate (text with tooltip) A cell plate is the type of cell wall formation that occurs in a centrifugal fashion. This occurs in some green algae, all embryophytes, and some of the brown algae. formation.
  • Meiosis: Present at least in Nephroselmis.
  • Sexual Reproduction and Life History: Confirmed in only one genus, Nephroselmis, which is isogamous (text with tooltip) Isogamous (adj) describes sexual reproduction in which the gametes are structurally identical. and haplontic.

• C. Ecology: Found in marine, brackish, and fresh water. Free-living and flagellated, or benthic with quadriflagellate zoospores.