24 



ACADEMIC BOTANY. 



tary, as in Navicula (Fig. 13, A). Each cell here also is 

 an individual. In different genera the cells are of different 

 shapes, — now like a rod (whence the name BaoUlaria, Little 

 Stick), now like a buckler (Fig. 14), now like a boat ; this 

 last getting its botanical name — Navicula (Little Boat) — 

 from its shape. The cell is called Frmtule (fragment), be- 

 cause at maturity the cells 

 separate from one another 

 as if broken at the joints 

 or points of union. Each 

 frustule is two-valved; it 

 imbibes silex, or silica 

 (flint, quartz), so that the 

 protoplasm is invested with 

 a shell. A drop of water 

 contains millions of these 

 tiny things ; when magni- 

 fied twenty diameters they 

 are still invisible. Yet 

 the silica protects them, 

 so that they are nearly in- 

 destructible. They live, 

 dried, a hundred years ; 

 then, when borne to the 

 water, or to a moist place, 

 they begin active life again. 



Their motions are wonderful. The Little Boats (Fig. 13, A) move 

 regularly back and forth as if propelled by an unseen oarsman with 

 invisible oars. And so they are ; the protoplasm 

 sends them to and fro in search of food ; and so 

 powerful is its attraction that the fine grains of 

 sand which furnish the silica of the outer wall of 

 the boat flock to it and run hither and thither along 

 its sides as if they too were alive. Un- _ 



der the cities of Kichmond and Peters- 

 burg, Virginia, there is a deposit of 

 their fossil remains twenty feet thick. 

 A cubic inch contains forty trillions ; 

 yet each tiny shell is carved with ex- „/'?■ M-— A, G^•ammotop*oramartIla; 

 '^ . .. . ■ rm. T» J.J. . Stoningtou, Conn. ; gait-water. B, 

 quisite tracery. The Kotten-stone, or meUtira sulcata ; a, frustule; Bich- 

 TripolijOf our household economy owes mond, Va. o, AciinoptjieimB smariut, 

 its polishing qualities to the shells of BichmoDd, Va. B and C are fossils, 

 myriads of fossil diatoms (Pig. 14). The Mountain-meal (Bergmehl) 

 of Norway, which the peasants mix with their flour in times of dearth 



Fig. 13. — A, Diatom {Navimla viridis). B, 

 Sesmid (Closterium aculttm)^ single individual. 

 C, same, two individuals coi^ugating. D, 

 same, spore formed ; greatly magnified. 



