These are microscopic single-celled green plants occurring both in the sea and in fresh waters. Many species form long chains of identical cells, and many have very attractive and intricate forms when viewed under the microscope. They are one of the more important groups of plants in the open ocean which provide directly or indirectly the food material for larger plankton and fish. The silica skeletons are sometimes deposited in enormous numbers on the sea bottom, forming diatomaceous earth. Such deposits are sometimes found on land, where the sea bed has been uplifted, and these are used both as a fine polishing grit and as the inert material in which nitroglycerine is absorbed to form dynamite. New Zealand geologists have recently begun to take an interest in fossil diatoms with a view to estimating the age of certain deposits and to determining whether they are of marine or freshwater origin.

by Richard Morrison Cassie, M.SC.(N.Z.), D.SC.(AUCK.), Senior Lecturer in Zoology, University of Auckland.