PLANT LIFE 145 



contents shrink into a dense mass in the middle 

 of the cell, and then develop a new and thick 

 wall within the old cell-wall, which is discarded 

 as soox^ as the resting spore is fully enveloped. 

 These spores, having a greater specific weight, 

 sink either into deep water or to the bottom 

 in coastal areas, where they lie for months 

 until conditions are again favourable for a 

 new start. The germination of these resting 

 spores has not yet been described. 



In the open ocean diatoms occur in greatest 

 abundance where there is an admixture of 

 muddy water from the land and where the 

 salinity is relatively low, for instance off the 

 embouchures of rivers, in the Indo-Pacific 

 region, where the largest rainfall in the world 

 occurs, and towards the Arctic and Antarctic 

 ice. Their abundance appears to be related 

 to the presence of colloidal silica or the 

 hydrated silicate of alumina in these areas 

 rather than to temperature, for they are found 

 abundantly in the tropics as well as towards 

 the poles. In the northern and tropical 

 Pacific, where the salinity of the surface water 

 is relatively low, all silica-secreting organism-s 

 are more abundant than in th^ Salter waters 

 of the Atlantic. It is well known that the 

 addition of some salt to muddy water at once 

 clears it by precipitating the clayey matter. 

 There is evidently a lack of silica in solution 



