12 DESMIDS AND DIATOMS. 



already noticed, but it becomes still more obvious in the two 

 tribes to which we now pass on, namely, the Desmidiacece and 

 the Diatomacece, or, as we will style them, the Desmids and the 

 Diatoms, in which stars, crosses, triangles, and other graceful 

 geometrical figures take the place of the more ordinary vegetable 

 forms, and give to these tiniest of plants the appearance of so 

 much mimic jewellery. 



The Desmids are green gelatinous bodies, confined exclusively 

 to fresh water. In most cases they consist of a single cell, which 

 is nipped in, in the middle, like a young lady with a small waist, 

 while the outer surface is frequently projected into spines and 

 variously-formed processes, which present a very symmetrical 

 and elegant appearance. It has recently been discovered that a 

 regular circulation of fluid takes place in the interior of these 

 minute organisms ; and it may interest the reader to know that 

 the gentleman to whom we owe most of our information on the 

 subject is no other than the redoubtable S. G. 0. of the " Times ' 

 newspaper, who, it appears, is wont to relieve his severer occupa- 

 tions by an occasional turn to the microscope, or the inmates of 

 the aquarium. The Desmids perpetuate their race in three 

 distinct fashions. In the first place, they multiply themselves by 

 the process of self-division ; secondly, by the breaking up of the 

 " endochrome," or coloured contents of the parent cell, into a 

 multitude of granular particles, which are set free by the rupture 

 of the cell wall, and then start in life on their own account ; and, 

 thirdly, by the process known as "conjugation," which may be 

 regarded as the first foreshadowing of the true sexual union which 

 takes place in the higher plants and animals. 



In many respects the Diatomacecp, have a close affinity to the 

 preceding group ; but they are unmistakably separated from 

 them by the possession of a bivalve shield, of pure colourless 

 silex, which renders their forms indestructible. It is to the posses- 

 sion of this silicious shield that the Diatoms owe all their beauty 

 and importance. There is a wide diversity of form in the shields 

 of the various species ; but in all of them alike the surface is 

 sculptured with a beautiful, well-defined, and more or less com- 

 plicated pattern of lines and points, which afford a ready means 

 of distinguishing the different species, and at the same time 

 places these minute bodies amongst the most elegant of micro- 

 scopic objects. In many of the species the individual Diatoms 



