l392.] NEW-VORK MICROSCOPICAL SOCIETY. 21 



up within. When binary subdivision takes place, the bottom and 

 the top of the box slide apart, while a new top is built on to the 

 old bottom and a new bottom on to the old top; — all inside of 

 the telescopic tubes just mentioned, which are simultaneously- 

 extended and duplicated by depositions of new siliceous material 

 upon the surface of the enclosed vegetating protoplasm. When 

 the new frustules are completely formed they are set free by the 

 slipping apart of the telescopic tubes, which, in the cas^ of the 

 free diatoms, usually fall away as genuine hoops, but in the case 

 of the filamentous forms more generally remain adherent to the 

 new frustules. 



I have spoken of the living substance of the diatom as being 

 sealed up within its glass case; but this view of the matter must 

 be taken with some qualification, since the vital functions could 

 not proceed unless there were communication of some kind 

 between the endochrome and its environment. The silex-coated 

 cell needs to be pervious to liquids and gases, as much as if it had 

 only a cellulose wall, like most other plant-cells. From what we 

 know of vegetable physiology generally, we do not run much risk 

 .in assuming that a constant interchange of elements takes place 

 between the active body within and the world of inert matter 

 without, by means of what is known as osmose. 



This presumption has furnished the basis for a third theory of 

 diatom-motion, which assumes that when endosmose occurs at 

 one end of the frustule exosmose occurs at the other, and that 

 the diatom is accordingly partly drawn and partly pushed along 

 by this sucking and ejecting operation. It is held that the flow 

 is not always in the same direction through the cell, but is subject 

 to alternation, which some observers believe to be at regular in- 

 tervals under all ordinary circumstances, though it is assumed to 

 be more likely that the direction, duration, and change of the 

 current depend upon some subtle and inconstant cause, like the 

 action of light or of heat. 



It is well known that all protoplasmic bodies are delicately 

 sensitive to external stimuli, of which light is one of the most 

 active and potent. The researches of Strasburger into "the 

 action of light and of heat upon swarmspores " have shown that 

 it is the actinic end of the spectrum which exerts the irritating 

 influence which results in their migration, while the green, yellow 



