14 OF THE FREE BIATOMACEiE. 



CHAPTER III. 



OF THE FREE DIATOMACE^E. 



The Diatomace^e may be conveniently divided into two 

 classes, the free species, or those which have an indepen- 

 dent existence ; and the stipitate, or such as are attached to 

 other objects, generally the larger Algge, by means of a 

 stalk. This kind of classification, though purely arbitrary, 

 is not without a certain importance, so far as the purposes 

 of collection and preparation are concerned ; for, as it is 

 one of the main points in a well-ordered herbarium to 

 retain the natural habit of each plant, we must not over- 

 look the various modes of growth to which the Diato^ 

 macese are subject. 



[Acting on the Horatian maxim, that 



* Segnius irritant animos demissa per aures, 

 Quam quae sunt oculis subjecta fidelibus,' 



I add here a few examples of the two classes of Diatoms 

 referred to above. Figs 25, 26, Plate v., represent members 

 of the l Free ' genera, Navicula and Pinnularia. 



In these it will be seen that each plant is a separate and 

 independent individual ; whereas in those which follow, 

 individuals are attached together, or to some other body, 

 either by a slender cord, or by a stem, from the ends or 

 sides of which they are produced ; in other words are 

 1 Stipitate : ' for these see Plates vi. and vn. 



A third form includes the Frondose species, or those in 

 which numerous individual frustules are enclosed in, and 

 held together by, a coating of gelatine; bearing, in this 

 state, a strong resemblance to the frond of a tiny seaweed. 

 Figs 31 to 34 (Plate vm.) belong to this group. — Ed.] 



