192 SEAWEEDS 



ever, been established by direct observation, is the 

 fact that most of the Diatoms possessing such a 

 power of movement possess also longitudinal sutures, 

 and it is supported by the observation that fine 

 particles of matter (not motile by themselves) sus- 

 pended in the water, on coming in contact with the 

 membrane, are set in motion along the suture. 

 However, no one has so far demonstrated the pro- 

 trusion of any protoplasm at this place. That these 

 movements take place with considerable relative 

 force is shown by the observation of Donkin, who 

 saw one species push away another at least six times 

 its size, while other observers state that they have 

 seen this greatly exceeded. The speed of the move- 

 ment when compared with the rapid dartings of 

 ciliated organisms is slow. The Rev. William Smith 

 estimated the rate at about four hundred times a 

 Diatom's length in three minutes. 



Reproduction. Diatoms multiply by successive bi- 

 partitions. The actively vegetating cell increases 

 slightly in volume, and the two valves are slowly 

 pushed out until the overlapping girdle remains in 

 that position, but no more. The protoplasm then 

 divides into two, and two daughter-cells are formed, 

 not separated by a membrane in the ordinary way, 

 but each new cell forms a new valve on the plane of 

 division. The edges of the new valves fit into the 

 girdle of the old valves of the parent cell, and these 

 old valves overlap the new ones. Each daughter- 

 cell thus possesses two valves, one newly formed and 

 overlapped by the old one. The cells either separate 

 and become free, or they remain in contact, and 



