NATURAL HISTORY OF THE DIATOMACE7E. 449 



ribbon-like assemblage of individuals result. The genera Fragilaria, 

 Himaiitidium, and Rhabdonema, are examples of such a mode of growth 

 when the valves are oblong, and Melosira when they are circular. Then, 

 again, the separation may be partial, so that the frustules remaining 

 united by the alternate corners are attached to each other, and a zigzag 

 chain is formed. Diatoma and Grammatophora are examples of this. 

 If the frustules are not possessed of quite parallel sides, but if they, on 

 the contrary, approach each other at one end, and then remain united 

 after subdivision has taken place, a fan-shaped arrangement will result, as 

 is seen in Lickmaphora ; or, if subdivision continues, a spiral will be 

 formed as in Meridion. Those forms which do not float about freely in 

 the water in which they live and grow, are attached to submerged objects 

 by either a small gelatinous cushion, a long single or branching stalk, 

 pedicle, or "stipes," as it is called. And there are forms, as Sdiizoncma, 

 which are of a naviculoid form, but which secrete around themselves a 

 membranous tube within which the process of subdivision goes on, and 

 up and down the cavity of which the little boats sail with extreme 

 activity. Thus, from this simple process of subdivision, as described, 

 various forms may result, and many individuals be formed which shall 

 have their number still further increased by the process of reproduction 

 to be next described. 



PART FOURTH. 



Reproduction of the Diatomace^e. 



We have seen the manner in which the diatomaceas increase in dimen- 

 sions, or grow, and observed that it is essentially the same process as 

 that which takes place in larger and apparently more complex organisms, 

 both animal and vegetable. That is to say, we have found that in the 

 case of the diatomaceae it becomes very difficult, if not impossible, to 

 distinguish the results of growth from the results of reproduction. And 

 although at first sight this may appear a very remarkable fact, yet it 

 ceases to be so if we remember that really physiology teaches us that 

 reproduction is merely a form of modified growth, resulting after the 

 casting off from the parent's body of one or more masses of matter which 

 possess in themselves the power of assimilation of food, and its appro- 

 priation for the building up and elaboration of new tissues. Those genera 

 vol. 1. 59 



