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Monterey Bay Aquarium Research Institute
Marine Botany

Ppericentral cellslysiphpericentral cellsnia

 Polysiphonia is a small alga; most of the species used in this study were approximately two to ten centimeters tall. Therefore, most aspects of its morphology are only visible under the microscope. Click on the links below to find out about aspects of Polysiphonia's morphology:

Polysiphonous Construction

Pit Connections

Apical Growth

Branching

Trichoblasts

Polysiphonous Construction

          Polysiphonous contruction is one of the most noteworthy characteristics of Polysiphonia. Apical cells cut off proximal segments which elongate to become axial cells. These cut off pericentral cells of the same length. The central axial cell is now surrounded by pericentral cells of the same length, forming one “tier” or segment. Polysiphonia can have four to 24 pericentral cells present per tier.

siphonous construction

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Pit Connections

          A pit connection consists of an opening in the cell walls of two adjacent cells which is filled with a plug and plug cap. Pit connections have a somewhat misleading name because there is not an actual connection or transport of material between the cells. Pit connections may lend support ato the thallus of the algae. The pit connections between pericentral cells and axial cells form during cell division and are referred to as primary. The pit connections between pericentral cells and other pericentral cells are referred to as secondary. This picture is an example of a secondary pit connection.

pit plug close up

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Apical Growth

branch tipThe dividing cells in Polysiphonia are localized in the apical regions. In this picture of the tip of a branch, you can see newly formed cells branching on the left, and slightly older cells beginning to form"tier" structures to the right.

 

 

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Branching

Polysiphonia  Polysiphonia has monopodial branching, which means that the primary axis remains the main line of growth throughout the life of the alga, with secondary branches forming off of the main axis. Branching is radial; branches form in multipe planes rather than in just one. Exogenous branches arise from segments immediately below the apical cell and cut off before pericentral cells cut off from the axial cell; these are more common in Polysiphonia than endogenous branches, which arise from the central cell after the pericentral cells have been cut off 

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Trichoblasts

          Trichoblasts are colorless hair-like filaments which cut off from the apex of the alga exogenously. On the left, you can see trichoblasts amongst some spermatangium on the apex of a male gametophyte. The sexual organs are borne on trichoblasts. Oftentimes, trichoblasts are deciduous, leaving behind scar cells (the trichoblast's basal cell). On the right, you can see scar cells between the polysiphonous tiers. Earlier, the trichoblasts existed in these positions and marked the apex of the alga. But since then, more tiers have been added and the trichoblasts have fallen off, leaving the scar cell.

trichoblastsscar cells

 

Trichoblasts (left)

 

Scar cells (right)

 

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This polychaete worm was about 1.5 inches long but could stretch another inch.