NATURE ADRIFT 



slightly unequal size, one being the same as the original, but the other, 

 formed from the previous inside half, will be slightly smaller. On re- 

 division the same thing occurs, and this can be useful information to the 

 marine biologist as the detailed examination of the size range of any species 

 in a diatom 'patch' in the sea helps him to identify the patch and gives a 

 clue to the number of generations that have gone into its making. When 

 cells become too small, they form auxospores which are aggregations of the 

 cell contents in a large thin-walled swelling formed from the parent ceU. 

 These grow and form much larger cells of the normal shape and so the process 

 starts again. These auxospores can be, but are not necessarily, involved in 

 the production of resting spores which can survive conditions, such as the 

 cold w'inters, which might be fatal to the normal cells. 



Some diatoms are almost always found singly, or in pairs immediately 

 after division (Frontispiece; 23, 24). Others cling in chains in which the 

 cells show several different types of attachment; closely joined by the cell 

 walls, interlacing of spines, ribbons of mucilage, or cytoplasmic threads 

 formed from the actual living cell structure. The chains may be simple like 

 strings of beads, ribbons coiled spirally or twisted in various directions, or 

 as rods attached at one end only or at alternate ends. A series of pictures of 

 some of the common types is given in the Frontispiece and a number of 

 these can also be seen and recognized in the photograph, Plate VII. 



Most of the diatoms are between about 1/200 inch and 1/50 inch, but 

 they range from about 2-5 fx (1/1,000 inch) to 1,800 jU. (1/14 inch). One of the 

 smallest that is abundant in the coastal waters around the British Isles in 

 spring is Skclctoiicnia costatiiiii (Frontispiece; 7) which has cells of about 7-15 /x 

 across. Later in the year we fmd Rhizosoleiiia stylijoriiiis, (Frontispiece; 18) a 

 large needle-shaped species that may be as much as 1/200 inch across and 

 1/20 inch long. An occasional visitor in our Atlantic water is Ethmodiscus 

 gazcllac, a relatively enormous round pill-box species similar to Cosciiiodiscus 

 illustrated in Frontispiece; i, but 14 inch in diameter. More about the 

 abundance and succession of these diatoms throughout the year is given in 

 Chapter 9. 



A diatom from eastern waters, Biddiilphia sinensis (Frontispiece; 25) named 

 from its Chinese origin, was found in the southern North Sea near Heligo- 

 land in 1903 and assiuned to have been brought in the ballast of some ship 

 to the Elbe. Finding conditions suitable it multiplied there and became quite 

 an abundant species in the southern North Sea and indeed was for a time 

 conveniently used by planktologists as an indicator of this water. It gradu- 

 ally spread farther and by now it has far outgrown this restriction and is 

 found in the English Channel, Irish Sea and northern North Sea. 



Because of the very fme sculpturing of the diatom frustules they have 



36 



