Article: Crystal development in Discoasteraceae and Braarudosphaeraceae (planktonic algae)

In both families the skeleton consists of little rosettes of calcite crystals grouped round a central axis. In the Discoasteraceae the crystals are arranged each with its optic axis parallel with the principal (= central) axis; the individual crystals have a bilateral symmetry with virtual suppression of the trigonal symmetry characteristic of inorganically grown crystals. The Braarudosphaeraceae differ in having their crystals arranged with a cleavage plane at right angles to the principal axis, to which the optic axes are oblique; the calcite is internally laminated parallel to this cleavage, and all three cleavages respond differently to etching. It is suggested that these peculiarities in morphology and chemical behaviour result from biochemical controls exerted by the organism during crystal growth, producing complex internal structures on a very fine scale, yet not fine enough to interfere with the crystal-lattice.