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This group of green algae lack motile gametes (sex cells), although sexual reproduction does occur. In the Connecticut River, two genera, Spirogyra and Sirogonium) have been found. These taxa consist of green unbranched filaments that often will form large, stringy, slimy masses. In the summer such algal blooms may cover sunken tree tops on the river bottom, forming large "hay stacks" of algae. The "hay stacks" are the favorite habitat for fish (especially catfish) that take cover in these masses of algae. These habitats disappear very quickly after rains, since they are fragile and are destroyed by fast water.

Spirogyra Link 1820

Unbranched filaments; cells are cylindrical to subcylindrical; usually they contain one to several partial spiral chloroplasts and contain pyrenoids spaced at regular intervals along their length.

Other species contain one axial, plate-like chloroplast or two massive chloroplasts, which are often more or less stellate. The nucleus, which is often clearly visible in the living cell, lies at the center of the cell, suspended by strands of protoplasm which traverse the large vacuole. Sexual reproduction occurs by conjugation.

CONJUGATING FILAMENTS

Sexual reproduction in Spirogyra. The formation of conjugation tubes (double-headed arrow) between cells of adjacent filaments (A, B) is followed by the movement of the contents of one cell (the male) into the other (the female) (C). Fertilization occurs (D) resulting in a diploid zygospore (single-headed arrow). In the Connecticut River, zygospores are found in the Fall and remain dormant until Spring.

Cell size and shape, pyrenoid shape and size, width and arrangement of chloroplasts, shape of spiral chloroplasts and cell wall structure have been used to identify six species of the genus in the Connecticut River. This genus is very sensitive to low pH. Cells were killed at pH 4.0 bur grow very well at pH 8.0.

Sirogonium Kuzing

This genus superficially resembles Spirogyra but its chloroplasts are long and thin and never form a helix. Several ribbonlike chloroplasts orient parallel to the long axis of the cell. Furthermore, the filaments lack the gelatinous investments characteristic of those Spirogyra.

The most important difference from Spirogyra is seen when they have sex when the gametangia conjugate directly without forming conjugation canals. The ameboid male gametes are smaller than the female's.