182 NOTES AND MEMORANDA. 



The specimens sent to M. Petit contained Melosira arenaria nearly 

 pure, containing only a few frustules of Surirelln spiralis and Epithe- 

 viia helvetica. It was easily seen at the first examination nnder the 

 Microscope that they were in full vital activity. 



Thus, according to these ohservations, diatoms continue to live, 

 and even to develop, in water at 0° with a surrounding temperature 

 of 9° to 18° below zero, provided always that they receive some rays 

 of light. 



M. Petit further says that " it is extremely curious to find at these 

 great altitudes species which are found in the plains ; it is impossible 

 to distinguish any difference between the Alpine species and the 

 others." 



Movements of Diatoms and Oscillatorieae. — The comparatively 

 rapid movements in the water of diatoms and of certain desmids, and 

 the wavy motion of the Oscillatorie^, are among the most familiar 

 phenomena to microscopic observers ; but their cause is at present 

 involved in much obscurity. 



Professor Engelmann, of Utrecht, who has undertaken extensive 

 observations on the subject, thus sums up our present knowledge.* 

 The most probable explanation at present offered, he considers to be 

 that of Max Schultze, f who attributes them to the movements of con- 

 tractile protoplasm which covers the outer surface of the solid cell- 

 walls ; a hypothesis which is confirmed by the following considera- 

 tions : — Diatoms exhibit this power of motion only when in contact 

 with a solid siibstratum ; they never swim freely through the water ; 

 which contradicts the hypothesis that the motion is due to vibratile 

 cilia or to osmotic currents. The phenomenon is especially marked 

 when they lie upon one of their so-called " sutures," and the motion 

 is always in the direction of this suture, either forwards or backwards. 

 Foreign bodies, such as grains of indigo or other pigments, easily 

 become attached to the surface when in contact with a suture, and are 

 moved up and down along it. This motion of the foreign particles 

 takes place only when they lie upon one of the sutures, and then 

 whether the diatom itself is in motion or at rest. 



In the case of Oscillatorieae, the following observations have been 

 made by Siebold : J — If the water in which these bodies grow is 

 coloured by indigo, the particles of this pigment which come into 

 contact with the separate OsciUaria-^li\.ments collect into a rather 

 narrow sj)iral running round the filament to its apex, whether the 

 filament is in motion or not. Sometimes these creeping spiral lines 

 of jjigment begin to be formed at both ends of the filament, and meet 

 in the middle, where the particles become heaped up into little balls ; 

 or sometimes they begin in the middle and advance to both ends of the 

 filament. The mode in which the particles of indigo adhere to the 

 alga and to one another appears in this case also to indicate an 

 excretion of mucilaginous protoplasm by the former. Cohn § subse- 



* ' Botanische Zeitung,' vol. xxxvii. (1879) p. 49. 



t ' Archiv fiir Blikr. Anat.,' (1865) pp. 376-402. 



X ' Zcitsch. fiir wiss. Zoologic,' vol. i. (1849) p. 284 ct scq. 



§ Cohn, in ' Archiv fiir Mikroskopische Anatomie,' vol. iii. (1867) p. 48. 



