I - The life history of some marine plankton diatoms

Abstract

The diatoms are divided into two big groups, the Pennatae and the Centricae. The first group consists mainly of bottom forms which are either attached or show free movement, whereas the non-motile, suspended Centricae form the main bulk of the planktonic diatoms. The present work is concerned with the second group only. A knowledge of the essential structure of the cell, which is very similar in both groups, is of great importance for understanding the life history of the diatoms. The cell walls consist of two parts which overlap each other like the two halves of a pill box; the valves forming the top and bottom, the connecting bands or girdle the sides of the box. In the course of cell division the two halves of the cell wall become separated and each daughter cell forms a new valve and new connecting bands to fit into the old girdle. Owing to the silicification of the cell membranes, growth is possible only along the pervalvar axis, the longitudinal axis of the “box”. Consequently the diameter of one cell is slightly smaller than that of its sister cell, i.e. by twice the width of the cell wall. In a population this peculiar mechanism of cell division leads to a continuous decrease of the average cell size.