326 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



Diatoms extract this silica from the water, where it is dissolved 

 in very small amounts, and deposit it in the membranes of their 

 cell walls to form the microscopic boxlike encasements or shells that 

 protect and support the living cells. 



The most striking features of a diatom, as seen under the micro- 

 scope, are the symmetry of its form and the intricacy and perfection 

 of structure of so minute an organism ; nor is this purely accidental, 

 for nature is a most careful and efficient designer. Not only is 

 this structure intrinsically beautiful and interesting, but it is of 

 great significance in several respects. 



Of course, the structure is primarily of fundamental importance 

 to the diatom itself; secondly, it is of importance in the general 

 economy of nature, insofar as this concerns the diatom ; and thirdly, 

 and incidentally, in the possession of properties different from those 

 of any other material, it is of wide general importance to mankind, 

 as we shall hope to see. It will be well to consider these three 

 important functions of diatom shell structure in the above order. 



1. IMPORTANCE OF SHELL STRUCTURE TO THE DIATOM 



Obviously, no active living organism could for long survive in a 

 single solid glass shell, for it could neither grow nor reproduce. 

 The rigid and inflexible nature of such a substance as silica necessi- 

 tates some special arrangement whereby growth and reproduction 

 of the cell may be accomplished, and nature has provided for this 

 by designing the cell wall or shell of the diatom in two parts like 

 a box, with a box and a lid, and a flange or girdle on each part to 

 overlap or telescope them together, a unique plan that is found in 

 no other organism. In fact the whole life of the diatom seems 

 so ordered as to be fitted to the nature of this material; and the 

 structure of the silica shell, in turn, gives constant evidence of its 

 adaptability to facilitation of the life processes of the cell. 



In growth, for instance, a cup- or box-shaped shell of so inelastic 

 a substance cannot expand in diameter, or length and breadth, as 

 can the ordinary plant and animal cell walls composed of more 

 flexible materials such as pectin, cellulose, protein, or other organic 

 material. The cell can grow only in depth, by a building onto of 

 the edges of the shell or girdle and a gradual slipping apart of the 

 telescoped shells, one from the other, as seen in plate 4. Thus the 

 cell can grow only by extension in one direction, or expansion in 

 one dimension instead of three, a phenomenon rather peculiar to 

 the diatom. 



In reproduction, when the cell has grown and matured as above 

 described, the protoplast within divides in two, and new shells are 

 deposited upon adjacent walls of the two new protoplasts thus 



