Abstract
An alternative approach to the exclusive use of fossils in determining polarity of character states in diatoms is presented. Principles of the method known as outgroup comparison are discussed. An example using features of sexuality and the relative number and morphology of chloroplasts is offered, and differences in polarity suggested by the fossil and outgroup methods compared. Based on the outgroup criterion, oogamy and many, small discoid chloroplasts found in most centric diatoms are considered derived, while isogamy and few large, plate-like chloroplasts found in many pennate diatoms are considered primitive. Although the centric diatoms may occupy a position that stratigraphically pre-dates the pennates, they appear to possess derived features of sexuality and chloroplasts.
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