22 INTRODUCTION. 



may be rendered apparent by taking a portion of mud covered 

 with Closteria, placing it in water exposed to light, and then 

 if the Closteria are buried in the mud they will soon work 

 their way to the surface, covering it again with a green stra- 

 tum*," I have myself frequently observed this fact, espe- 

 cially in Tetmemorus granulatus and Penium Brebissonii ; but 

 I presume that it is owing to the stimulus of light rather 

 than to any voluntary effort ; at any rate the same result w^ill 

 follow if an Oscillatoria be substituted for the Closterium. 

 In like manner, in some species of Nostoc, Anabaina, and 

 Palmella, the filaments or cells throng towards the light ; and 

 should the specimen be turned over, they will in a few hours 

 appear on the new surface, whilst they have become less nu- 

 merous on the previous upper one. Of course the gelatine 

 must be in a sufficiently lax state to permit such a movement. 



Another proof of their power of locomotion is afforded by 

 their retiring, in some instances, beneath the surface when 

 the pools dry up. I have taken advantage of this circum- 

 stance in order to obtain specimens less mingled with foreign 

 matters than they would otherwise have been. If a species 

 be much mixed with mud, I take a saucer, fill it with earth 

 made into a paste with water, and cover it with a piece of 

 linen ; over this I spread a thick layer, containing the Des- 

 midiese, and allow it to become nearly dry ; within a few 

 days the specimens will form a stratum on the linen, and may 

 be scraped off with a knife. This plan however proves suc- 

 cessful only with the smaller species, and minute Algae ac- 

 company the Desmidiese which are so obtained. 



It is thus evident that whatever be the motive power of the 

 Desmidiese they possess it only in common with acknow- 

 ledged Algae, and in a less degree than either the Diatomacese, 

 the Oscillatorieae, the sporules of various Algse, or indeed their 

 own sporules. 



Ehrenberg considers that increase by voluntary division is 

 the character which separates animals from vegetables!, and, 



* American Bacillaria, in American Journal of Science and Arts, vol. xli. 

 p. 300. 



t Annals of Natural History, vol. ii. p. 123. 



