148 A TEXTBOOK OF THEORETICAL BOTANY 



half has therefore one valve of the old cell and one naked side, which it covers 

 by secreting a new valve fitting iinide the old one. One daughter cell there- 

 fore remains the same size as the parent, but the other, which took the smaller 

 valve with it, is slightly smaller. Thus in successive cell divisions one line of 

 daughter cells becomes progressively smaller. As this cannot go on indefinitely 

 a restorative process eventually occurs. The cell secretes abundant mucilage, 

 which forces the valves apart, and the liberated protoplast grows to the 

 maximum size of the species and then forms two new valves which reconstitute 

 a new cell. This liberated protoplast is called an auxospore. 



In many other Diatoms auxospore formation is associated with con- 

 jugation, but sexual reproduction is not known to occur in Pinmdaria. In 

 related forms, e.g., Cymbella, two individuals cohere and become enveloped 

 in mucilage (Fig. 138). In each cell the nucleus divides into four, presumably 

 with chromosome reduction, and two of the nuclei in each cell abort. Each 

 protoplast then divides into two and fusion takes place, by two and two, 

 between the conjugating protoplasts. The two fusion cells formed then 

 function as auxospores and secrete new frustules. 



Reference has been made above to the probable occurrence of meiosis 

 preceding conjugation. Although this has not been definitely proved in all 

 cases it is generally believed that the cells are diploid, as no reduction is known 

 to occur after conjugation. Somatic divisions are mitotic and follow normal 

 lines, except for the presence of a centrosome from which the spindle 

 originates. 



Pinmdaria shows a power of creeping movement on a solid substratum 

 which is well seen in specimens mounted on a glass slide. This seems to be 

 due to protoplasm extruded through the raphe and circulating along the 

 surface of the valve, returning through openings in the polar nodules, thus 

 providing a sort of gastropodous movement, rather like that of a tank-track. 

 The motion of the protoplasm can be demonstrated by mounting the cells in 

 a suspension of Indian ink or Carmine. The external protoplasm may be 

 in two layers, as opposing currents of granules can be seen on the same 

 surface. 



Plankton 



Mention has been made of the importance of the Diatoms as constituents 

 of the marine plankton. They are also abundant in fresh-water plankton, 

 though this is of minor importance compared with that in the sea. In coastal 

 waters their development follows a well-marked annual cycle. In April 

 occurs a great vernal outburst of Diatom life, which rapidlv depletes the 

 reserves of phosphate, nitrate and dissolved silica in the sea water and leads 

 to a midsummer dearth. Meanwhile the dead cells have sunk and decayed 

 in deeper waters, releasing their stores of absorbed nutriment. Some of this 

 is restored to the surface by the disturbance of autumnal gales, producing 

 a minor secondary maximum in September and October. This is followed 

 by another minimum during the winter. Meanwhile the chilled surface 



