DOES THE DIATOM MOVE IN DARKNESS? 



BY T. CHALKLEY PALMER. 



In a previous paper in these Proceedings * certain exper- 

 iments on moving naviculoid diatoms were described. The 

 conclusion drawn from these experiments was adverse to the 

 notion that light (at least such light as was used in the tests 

 described) exerts any necessary attractive force upon the 

 motile diatom, since all the diatoms studied moved in total 

 disregard of a lighted area in the midst of a semi-dark field. 

 Moreover, doubt was expressed as to the correctness of the 

 further notion — somewhat prevalent with theorists — that 

 diatoms are unable to move in darkness. This doubt is not 

 the fruit of a disbelief in the " osmotic " hypothesis. On the 

 contrary, the writer believes that hypothesis to be unnecessary, 

 erroneous, and worse than valueless because of facts already 

 known. But it may not be out of place to slay the slain once 

 more b}^ a demonstration that diatoms do move in a darkness 

 that is to all intents complete. 



The observations now to be described were made at night 

 with the aid of the following simple arrangements : — 



A small Beck binocular stand was fitted with a revolving 

 diaphragm in a substage tube, the arrangement being such 

 that light coming from below the stage could be admitted or 

 entirely excluded at will, and with rapidit}'. Cuplike caps, 

 impervious to light, were provided for the eye-pieces, and 

 these, when in place, cut off any faint illumination from 

 above. A silk cloth was folded into several thicknesses, until 

 no light could be seen through it when looking directly toward 

 a bright Welsbach lamp. This was passed around the tube 

 of the objective and secured there, the edges being brought 

 down around the circumference of the circular stage. No 

 light from any direction could now reach the stage except as 



*" Some 01)servations on Diatom Motion," Vol. II, No. 4, Jnly, 1907. 



