42 



BOTANY. 



and salt, forming usually, slimy, yellowish coatings on stones, 

 mud, aquatic plants, etc. Like the desmids they may be sin- 

 gle or united into filaments, and not 

 infrequently are attached by means of 

 a delicate gelatinous stalk (Fig. 25). 



They are at once distinguished from the 

 desmids by their color, which is always 

 some shade of yellowish or reddish brown. 

 The commonest forms, e.g. Navicula (Fig. 

 24, (7), are boat- shaped when seen from 

 above, but there is great variety hi this 

 respect. The cell wall is always impregnated 

 with large amounts of flint, so that after the 

 cell dies its shape is perfectly preserved, the 

 flint making a perfect cast of it, looking 

 like glass. These flinty shells exhibit won- 

 derfully beautiful and delicate markings 



which are sometimes so fine as to test the best lenses to make them 



out. 



This shell is composed of two parts, one shutting over the other like a 



pill box and its cover. This arrangement is best seen in such large forms 



as Pinnularia (Fig. 24, A n). 



Most of the diatoms show movements, swimming slowly or 

 gliding over solid substances ; but like the movements of Oscil- 

 laria and the desmids, the movements are not satisfactorily 

 understood, although several explanations have been offered. 



They resemble somewhat the desmids in their reproduction. 



FIG. 25. Diatoms attached 

 by a gelatinous stalk, 

 x 150. 



THE TRUE BROWN ALG^E. 



These are all marine forms, many of great size, reaching a 

 length in some cases of a hundred metres or more, and show- 

 ing a good deal of differentiation in their tissues and organs. 



One of the commonest forms is the ordinary rock weed 

 (Fucus), which covers the rocks of our northeastern coast with 

 a heavy drapery for several feet above low-water mark, so that 



