Size Range And Diversity Of Structure

The size range of the algae spans seven orders of magnitude. Many algae consist of only one cell, while the largest have millions of cells. In large, macroscopic algae, groups of cells are specialized for specific functions, such as anchorage, transport, photosynthesis, and reproduction; such specialization indicates a measure of complexity and evolutionary advancement.

The algae can be divided into several types based on the morphology of their vegetative, or growing, state. Filamentous forms have cells arranged in chains like strings of beads. Some filaments (e.g., Spirogyra) are unbranched, whereas others (e.g., Stigeoclonium) are branched and bushlike. In many red algae (e.g., Palmaria), numerous adjacent filaments joined laterally create the gross morphological form of the alga. Parenchymatous (tissuelike) forms, such as the giant kelp (Macrocystis), can measure many metres in length. Coenocytic forms of algae, such the green seaweed Codium, grow to fairly large sizes without forming distinct cells. Coenocytic algae are essentially unicellular, multinucleated algae in which the protoplasm (cytoplasmic and nuclear content of a cell) is not subdivided by cell walls. Some algae have flagella and swim through the water. These flagellates range from single cells, such as Ochromonas, to colonial organisms with thousands of cells, such as Volvox. Coccoid organisms, such as Scenedesmus, normally have an exact number of cells per colony, produced by a series of rapid cell divisions when the organism is first formed; once the exact cell number is obtained, the organism grows in size but not in cell number. Capsoid organisms, such as Chrysocapsa, have variable numbers of cells. These cells are found in clusters that increase gradually in cell number and are embedded in transparent gel.