NATURE ADRIFT 



variations (Plate I). The shells (Plate VIII) are sculptured in various ways by 

 the addition of spines and small holes— 'puncta' — in strict patterns, through 

 which the internal cytoplasm of the cell, the living part, has contact with the 

 water outside. Some diatoms, chiefly those that live on the bottom, have one 

 or both valves pierced by a slit called a raphe (Fig. 9) and these forms can 



raphe 



Fig. 9. A naviculoid diatom, Piimnlaria, showing raphe, X 1200 diameters. 



move slowly along, invisibly propelled by slight pressure differences caused 

 by the minute chemical differences between the two ends of the cell. The 

 movement is reminiscent, in fact, of the toy paper boats propelled by adding 

 a small piece of naphtha into a slot in the deck. These kinds of diatom are 

 sometimes carried into the plankton from the bottom by turbulent currents 

 or wave action near the shore and will be amongst the most frequent kind 

 if samples arc collected from rock pools in the way described on page 14. 



Diatoms have been abundant since Cretaceous times and their shells of 

 silica have fallen to the sea bed, gradually accumulating through geological 

 and recent times to form a diatomaceous ooze which predominates in a 

 virtually continuous belt around Antarctica and in a band across the North 

 Pacific Ocean. The dried ooze is also mined in places now above sea level 

 and used as a polishing and insulating material. The most famous are the 

 Kieselguhr mines of Germany; and there is a flourishing mine in the Heb- 

 rides in Scotland, only opened in i960, although an older one in Skye 

 closed in 1959 as to continue mining there became uneconomic. There 

 are many others dotted over the world. Other planktonic marine organisms 

 also form oozes on the sea bed and these will be dealt with later in the 

 book. 



But to return to the living diatoms! They are mostly (and the plank- 

 tonic ones all are) holophytic or autotrophic, i.e. they live entirely by the 

 process of photosynthesis using dissolved salts and gases and utilizing the 

 sun's energy. This produces a large proportion of carbohydrate, with oils 

 and proteins. As long as there is suflicient food present as dissolved salts, 

 and sufficient light, diatoms continue to thrive and reproduce. This they do 

 by simple division, the two parts of the pill-box separate and each grows a 

 new half inside (Fig. 8). It thus follows that the two daughter cells are of 



34 



