450 PHYSICAL GEOGRAPHY. 



of diatomacese which occur normally, or, we should say, most com- 

 monly, attached in some manner, as by a cushion, pedicle, stipes, or the 

 surface of the valve or connecting membrane, or otherwise, to submerged 

 objects, would never become very widely distributed through the agency 

 of self-fission alone, as it has been described, and the consequence would 

 be that they would be confined to certain localities were there not some 

 other mode of increase or reproduction. To a certain extent, this distri- 

 bution is provided for by the curious movements of the individual which 

 we have just treated of, and which we have seen are quite lively in some 

 species. But it is still more perfectly insured by the process of repro- 

 duction in which a new individual is developed from a parent. It is in 

 the form of spores or seeds that most plants (or, at least, the larger ones 

 with which we are acquainted) are enabled to endure the severe frosts of 

 the winter months ; and the same is likely to be the case with the dia- 

 tomacese, although it is true that some species are to be found living and 

 swimming actively about beneath the frozen surface of ponds and streams. 

 And, although they be caught within the mass of solid ice, yet their 

 vitality does not seem to be materially compromised, for, on thawing the 

 ice they again move about in a lively manner in the water formed. Very 

 little investigation has been carried on in the direction of the reproduc- 

 tion of the diatomacese, or, rather, we should say, that little has been 

 published in this connection, so that we have few authorities to draw 

 upon to enlighten us on this point in the economy of our. little friends. 

 From what little has been observed and recorded by a few investigators, 

 it would seem that the diatomaceae reproduce after a manner very similar 

 to that which has been found to take place in the Protophyta, or simple 

 unicellular plants ; and this fact has been brought forward as an argu- 

 ment in favor of the vegetable nature of the organisms of which we 

 are treating. 



The first instance in which the process of reproduction was observed 

 and published was by Mr. Thwaites, in Epithemia, a genus which is 

 almost always found in the living state, attached to submerged plants, as 

 mosses and the like. He found it to be essentially the same as the mode 

 of conjugation, as it had been called, known to take place in several algae 

 or water plants of simple organization. He describes it in the following 

 manner: "The process of conjugation consists in the union of the endo- 



