Xii DIATOMACE.E OF NORTH AMERICA. 



Possibly this structure pertains to all diatoms, though it can be 

 detected only in the larger specimens. 



MOVEMENTS OF DIATOMS. 



These have excited the curiosity of all observers and given 

 rise to a belief with many, that the Diatom belongs tofche animal 

 rather than to the vegetable world: but though the latter 

 opinion now prevails, yet the cause of the movement of Diatoms 

 has not thus far been satisfactorily accounted for even by those 

 who have, like H. L. Smith, devoted much attention to the sub- 

 ject. This gentleman, at the tenth annual meeting of the Amer- 

 ican Society of Microscopists. presented ''J. Contribution to the 

 Life History of the DiatomacecE^ in which he says: "I am dis- 

 posed to consider" .for reasons he had previously stated at 

 length) "that the motion of the Xavicule;e is due to injection and 

 expulsion of water, and that these cm-rents are caused by different 

 tension of the" (internal "membranous sac in the two halves 

 of the frustule. In those Diatoms which do not have the central 

 band thus binding the frustules, as for example, the Si/nertrir, 

 the Fr(tf/itfiri<t' and the circular and angular forms, no motion is 

 to be observed, or at best but a slight trembling, as in this case. 

 the tension is more nearly uniform over the whole surface of the 

 internal sac.-' More recently, Mr. Cornelius Onderdonk, in The 

 Microscope for August, 1890. ascribes the movements of Diatoms 

 to "a thin fluid mass in rhythmical motion," which covers the 

 surface of the Diatom. "We cannot conceive that the fluid has 

 power to drive itself, for it is unformed material. What then 

 remains except that the motive force is on the walls of the 

 cell " 



REPRODUCTION OF DIAToMS. 



Diatoms reproduce themselves by three methods of conjuga- 

 tion. 



1st. By two frustules uniting their undifferentiated endo- 

 chrome. thereby producing a single sporangium about double 

 the size of the parent frustules. which finally develops into a 

 frustule with ends more rounded than the parent frustules and 

 the valves not so wedge-shaped. Xnr/xr//<t .^Ictnliilti is a good 

 example of this method, see Plate I. 



I'd. By two frustules uniting their differentiated eudochrome 

 and producing tiro sporangia! cells; sometimes one of these 

 proves abortive, but that has no influence on the growth and 

 development of the other. Xan'mhi (iiii/>ltii'/ii/nr/tix is propaga1c<| 

 in this manner. 



