DIATOM MOTION. 73 



slide was now strongly illuminated for an hour, but not a 

 single Navicula moved. A careful examination showed that 

 every NaviaUa in the slide was dead and the endoplasm much 

 contracted. Whatever the cause of the catastrophe, it had 

 not harmed the Nitzschi(c, neither had the long continued 

 darkness influenced their motile activity to a perceptible degree. 

 At this point it was thought best, for the sake of definite- 

 ness, to map the further observations, as in a former paper, 

 and a number of diagrams of the field, drawn to scale, were 

 prepared. On these were dotted the positions of the diatoms 

 observed, and the directions of motion at times of observation 

 were indicated bv arrows. 



Experiment III. — A few of the diatoms (perhaps not 

 more than loo) were put into a Holman life slide, and kept in 

 dense obscurity 24 hours. The slide was then momentarily 

 illuminated, and showed (See Figure i) three diatoms at loca- 

 tions I, A, B and C. After three minutes of total darkness 

 the diatoms had all moved to 2, A, B and C. These were all 

 Navicu/a gihba. The distance moved in three minutes was 

 approximately measured and found to agree ver}^ well with 

 the speed for this species as given in the former paper. 



Experiment IV. — (See Figure 2). Another mount of the 

 diatoms was made, but of such a sparseness that it was pos- 

 sible to find a field showing not a single frustule. The slide 

 was kept in darkness on the stage one hour. At the end of 

 this time it was observed at intervals of three minutes, with an 

 exposure of not more than 15 seconds each time, as follows : — 



8.47 No diatoms. 

 8.50 " 



8.53 " 

 8.56 " 

 8.59 " 



