328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



time to time as the cell grows, it is clear that these would be weakened 

 if the open side of each ring came at the same place as the one next to 

 it, making a continous cut across the whole series of rings ; so nature 

 avoids such a weakness by ordering that the opening of each ring 

 shall come at a different place in the series, like the mortar joints of 

 brick in a wall, no two being in line with each other. Nor is this 

 enough to satisfy the niceties of nature, for frequently the open ends 

 of these girdle bands are especially dovetailed and grooved to fit into 

 each other and lock together, and the edges of the girdle band or 

 ring are serrate or notched to fit in corresponding notches in the 

 rings adjacent to it (pi. 2). All of this adds, not only to the strength, 

 but also to the beauty of the diatoms in varying their patterns. And 

 when any particular feature of design of an object adds to its 

 strength, it usually means an economy in the amount of material 

 necessary for its construction, a matter of importance in the efficiency 

 of nature as will be noted later. 



To breathe and obtain food substances for growth and reproduc- 

 tion all cells must be able to absorb material in solution through 

 their cell walls, but as silica is an impermeable material, special pro- 

 vision must be made in the diatoms for such absorption. This ab- 

 sorption is made possible by numerous pores through the shells, vari- 

 ously arranged in the different species, frequently forming beauti- 

 ful patterns on the shells. These porous spaces are covered with a 

 semipermeable membrane, composed of some pectic substance, which 

 permits the interchange of liquids and soluble food materials between 

 the exterior and the interior of the cell. 



These porous spaces through the silica shell naturally lighten and 

 weaken the wall, and this weakness is overcome by various means 

 in different types of diatoms. Sometimes, in the more plain and 

 simple types of shells, there is merely a thickening of the silica 

 between the porous spaces. Seldom is the shell surface of the diatom 

 flat ; instead there is usually some degree of curvature to the surface, 

 often very slight, but in many cases the shell is highly concave or 

 convex, and this curvature in itself serves to strengthen the shell, just 

 as dome-shaped ceilings over large rooms can be supported more 

 easily and rigidly than flat ones. The flexible nature of the proto- 

 plast and the conditions of tension and of internal and external pres- 

 sure of the cell as the shells are being formed control these forms in 

 the various species. In many other diatoms — those with particularly 

 flat shells such as Arachnoidiscus, or others with large openly porous 

 shells such as Isthmia and many Biddulphias — internal ribs or cross 

 walls are present to add strength to the shell, just as ribs on large steel 

 doors and gratings, inside of pianos, and in the bottoms of boats are 

 used to increase strength instead of building a heavier solid structure. 



