Links With the Past in the Plant World

by A. C. Seward

Fig. 7. Flood-plain of the delta of Rio Colorado. The hills in the background are 25 miles distant. (From a photograph by Prof. MacDougal.)

Pieces of plants embedded in sandy sediment, if not preserved by petrifaction, that is by the introduction into the tissues of some siliceous or calcareous solution, gradually decay and their fragmentary remains may be washed away by percolating water, leaving a hollow mould in the gradually hardening sediment, which is afterwards filled with sand or other material. The plant itself is destroyed, but a cast is taken which in the case of fine-grained sediments reproduces the form and surface-pattern of the original specimen. The incrustation of plants by the falsely named petrifying springs of Knaresborough and other places illustrate another method of fossilisation.

Fig. 8. Flower of Cinnamomum prototypum Conw. preserved in amber. × 10. (After Conwentz.)

Plants which owe their preservation to amber occur both as incrustations and petrifactions. This fossil resin occurs in Tertiary, Cretaceous, and Jurassic rocks; the amber found in abundance on the Baltic coast near Danzig and occasionally picked up on the beach in Norfolk and Suffolk comes from beds of Tertiary age. Pieces of Pine-wood have been described from the Baltic beds in which the tissues are perfectly preserved as the result of the conversion into amber of the resinous secretion which permeates their cells: in this case the amber is a petrifying agent. More frequently the preservation is due to incrustation; as resin trickled down the stems of the Tertiary pines from an open wound, flowers and leaves, blown by the wind on to the sticky surface, were eventually sealed up in a translucent case of amber. Though the actual substance may have gone, the mould which remains exhibits in wonderful perfection each separate organ of a flower or the delicate hair-clusters on the surface of a leaf. The flower represented in [Fig. 8], a species of Cinnamon, is one of several specimens described by the authors of a monograph of Tertiary plants in the Baltic amber([41]).