Ray tracheids are present normally in Pityoxyla from the Middle Cretaceous on, and in the Abietineae. 2. Ray tracheids are present traumatically in the Taxodineae and the Cupressineae. 3. On the evidence of traumatic recapitulation of ancestral characteristics, it is evident that the Taxodineae and Cupressineae are descended from the Abietineae, having sprung from that line at some time after the Middle Cretaceous. 4. Since ray tracheids are universally absent in the Podocarpineae, Taxineae, and Araucarineae, these lines must have come off from the Abietineae at some time before the Middle Cretaceous. 5. These conclusions are corroborated by a study of geological records and of other anatomical structures, as resin canals, wood parenchyma, etc.
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Ruth Holden (1913) studied this question.