30 PLANT-LIFE 



faces of the valves, and each half of the cell, now become 

 an individual, retains one of the valves, and completes 

 its integument by forming a new valve to fit inside that 

 retained from the parent cell. Thus the two valves of 

 a Diatom resulting from division are invariably of 

 different ages, and we can see, moreover, that repeated 

 division inevitably leads to diminution in size of the 

 daughter-cells, and if the process were to go on indefi- 

 nitely, it would be extremely unsatisfactory. The cells 

 would get smaller and smaller, and ultimately so small 

 that the plants would be threatened with extinction. 

 But division and its consequent reduction in size proceed 

 to a certain length, when the process is arrested. Growth 

 and a return to normal proportions is provided for by 

 the formation of auxospores (Gr. auxe, growth). The 

 cell contents mass together, free themselves from their 

 valves, grow considerably, and finally secrete new 

 valves. These valves are thus of the same age as each 

 other. 



So far there has been no sign of sex, and, be it said, 

 sex is not conspicuous among the Diatoms. Neverthe- 

 less, it does appear, for it sometimes happens that two 

 cells conjugate, and unite to form a single auxospore; 

 and over and beyond this evidence of sex, it may also 

 happen that two cells may divide each into two daughter- 

 cells, which conjugate in pairs, the result being the 

 formation of two auxospores. Yet we note that, 

 although sex appears among the Diatoms, there is no 

 distinction of sex; we cannot say of two conjugating 

 cells which is male or which is female. 



Diatoms play an important part in the economy of 

 the life of the sea, as they form a food-supply for hosts 



