330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193G 



ocean have shells as lightly silicified as possible, and in order that 

 they may have the necessary strength they are usually very smvill, 

 round, or tubular in shape, or with numerous cross walls (pis. 2 

 and 5). Often, too, long spines increase the amount of surface to 

 keep them afloat. 



2. IMPORTANCE OF DIATOM SHELL STRUCTURE IN THE GENERAL 

 ECONOMY OF NATURE 



With all these various types of structure, and with their small but 

 rugged shells which because of their minuteness are capable of rapid 

 reproduction and may easily be transported hither and yon, it will 

 be realized how readily diatoms can be scattered throughout the 

 oceans and utilize to the fullest extent the dissolved mineral substance 

 present. 



These types of structure provide simultaneously for great st rength 

 with correlated economy in the use of silica, which because of its 

 high degree of insolubility in water (only a few parts per million in 

 most natural waters) is a precious material to the diatoms and is 

 often completely used up by them during the height of their growing 

 season. The porous and chambered type of w^all structure mentioned 

 heretofore as common in so many of the diatoms permits of the walls 

 being of exceeding thinness, in most cases beyond the possibility of 

 convenient measurement, and often as thin as two one-hundred- 

 thousandths of an inch, yet of great rigidity in order to avoid 

 collapse of the cell. It is advantageous in the economy of nature 

 that a great abundance of diatoms should be produced in all waters 

 to afford a food supply for the animals living there, even with the 

 small amount of silica available. It is likewise desirable in the 

 economy of nature that these diatoms should use sparingly of so 

 rare a substance as the slightly soluble silica, so that the available 

 supply will suffice to produce the greatest possible immber of them. 

 This could not be better accomplished than by the nicety of adapta- 

 tion exhibited by the very strong and yet very delicate structure of 

 their shells. Studies of this subject have been made in our labora- 

 tory and will be published more extensively in a later paper. 



Man uses the same methods in the construction of long bridges, 

 deriving great strength simultaneously with lightness of weight and 

 a minimum use of costly material by designing long open-work 

 girders and trusses. Another example of achievement of great 

 strength through the form of structure with even a very frail ma- 

 terial is the use of the common corrugated paper box for shipping 

 surprisingly heavy and extremely fragile materials. 



Another well-adjusted arrangement in the economy of nature is 

 the production of light-weight shells adapted for buoyancy of 



