208 BULLETIN OF THE BUREAU OF FISHERIES. 



a rate of 3 mm. per minute it will produce a velocity of movement of about 1 mm. per 

 minute. 



Light has a slight effect on the response of diatoms. In general the chlorophyllous 

 forms appear to be positively phototropic, while the colorless ones are negative. The 

 orienting action, however, is feeble, and the oscillating forms follow irregular paths 

 toward or away from the light. In the presence of light of moderate intensity negative 

 forms creep into the mud. 



The minimum temperature which diatoms can withstand varies considerably with 

 the species. Some have been reported to withstand — 200 ° C, but, in general, actively 

 vegetating forms are killed by freezing at — 8 to — io° C. In southern Newfoundland, 

 where the Labrador current mingles with the warm water of the Gulf stream, a great 

 mortality of a certain species, Cosinodiscus radiatus, has been taking place, for in this 

 region enormous quantities of the dead shells are found covering the bottom. 



Reproduction is by means of simple cell division or by the formation of what is 

 called an auxospore. In the first case the nucleus and protoplasm divide into two 

 parts, the valves separate, and a new valve develops within each of the old. The valves, 

 once formed, are fixed in size which determines that each successive generation will 

 become smaller. This constant decrease in size is compensated for by the formation of 

 the auxospore. In doing this the diatom leaves its shell, swells up to the maximum 

 size, and secretes a continuous membrane about itself. Within this there is first formed 

 a single valve, like one of the original ones, and soon after a second one fitting into it. 

 Sometimes the naked protoplast of two cells may escape and fuse together as one, in 

 true sexual union. From the cell thus produced a diatom is either formed at once or 

 after a preliminary division of the protoplast. The rate at which this process takes 

 place varies with the conditions, but, on the whole, we know it must be exceedingly 

 rapid in order to keep almost every part of the sea in the constant condition of a living 

 broth. Being bathed in a uniform medium containing dissolved nourishment and sub- 

 ject to the full benefit of the sunlight without being exposed to extreme changes of 

 temperature, growth and reproduction become so rapid that they pass beyond our powers 

 of conception. As the late Prof. Brooks has written: 



Their vegetative power is wonderful past all expression. Among land plants, corn, which yields 

 seed a hundredfold in a single season, is the emblem of fertility, but it can be shown that a single marine 

 plant, very much smaller than a grain of mustard seed, would fill the whole ocean solid in less than a 

 week if all its descendants were to live. This stupendous fact is almost incredible, but it is capable of 

 rigorous demonstration, and it must be clearly grasped before we can understand the life of the ocean. 



This wonderful productive power, together with their chemical composition, show 

 that they occupy an exceedingly important place in the economy of nature. Brandt 

 (1898) analyzed several samples of plankton. One, consisting chiefly of Chaetoceros, 

 gave the following composition: Albumin, 10 to 11.5 per cent; fat, 2.5 per cent; carbo- 

 hydrate, 21.5 percent; and ash, 64.5 to 66.0 per cent (50 to 58.5 per cent SiO,) . Another 

 sample, consisting of Ceratium tripos, had a very different composition, as follows: Albu- 

 min, 13 per cent; fat, 1.3 to 1.5 per cent; carbohydrate, 80.5 to 80.7 per cent; and 

 ash, 5.0. As Brandt points out, these results compare very favorably with those of 

 analyses of land crops. 



