DIATOMS— CONGER 329 



In many diatoms an alternate raising and lowering, or undulation, 

 of the surface gives strength and rigidity, just as is accomplished 

 by the corrugated surface of the galvanized sheet iron commonly 

 used for roofing, sides of garages, and factory buildings. (See pis. 

 14, 15, and 16.) 



In still another large series of rather more elaborate and complex 

 diatom shells, the shell surface consists not of one, but of two very 

 porous layers one above the other with very thin upright joining 

 walls between them forming small hexagonal chambers, into each 

 of which one pore of the external and one of the internal wall open, 

 forming thus, except for the previously mentioned permeable mem- 

 branous covering, a continuous passage through from the outside to 

 the inside, and yet giving a maximum degree of strength and great 

 rigidity to a highly porous surface by the use of a minimum amount 

 of material (pi. 3). In many of the larger-celled diatoms, in whicii 

 the mass of the cell is great in proportion to the material substance 

 and strength of the shell, the shells tend to assume the spherical 

 and tubular shapes — shapes much used by man for tanks, pipes, and 

 containers because these shapes are intrinsically strong and afford 

 the enclosure of a large amount of space with a minimum amount of 

 structural material. 



The fraility of this exceedingly delicate shell necessitates in many 

 species special types of strengthening and supporting structures, such 

 as internal cross walls or septae, thickened solid silica bars, and 

 sometimes even internal plates or chambers, or secondarj^ and inner 

 shells. All of these features, together with the natural trends of 

 variation, tend to nmltiply greatly the types of structure and form 

 found in the diatoms, so that they include every conceivable geomet- 

 rical design and variation and sometimes even almost unexplainable 

 shapes. 



Having seen! how important the structure of the shell is to the 

 diatom cell itself, if we turn now to consider how the structure re- 

 lates to the kind of place in which a certain species may live, we find 

 again an amazingly beautiful series of adaptations to fit the cell to 

 life in its particular habitat. Thus diatoms that grow attached to 

 other submerged plants or objects may take long needle-shaped 

 forms, allowing for the attaclmient of great numbers of their ends, 

 and their exposure of a maximum amount of surface to the water. 

 Another arrangement for this same purpose is found in fan-shappd 

 cells set in fan-shaped colonies attached by their small ends, and 

 in the growth of long filaments with endwise attaclmient. Large 

 free-living cells must have walls of heavier silica and often more 

 mternal cross walls. Since silica is twice as heavy as water, it is 

 necessary that diatoms whicli live continuously floating in the open 



