MICROSCOPIC FORMS OF VEGETABLE LIFE. 



279 



represent. By Mr. Stephenson, who has made a special study of the 

 effects of the immersion of Diatom-valves in very highly-refracting media, 

 it is believed that the light spaces really represent apertures ( 289). 

 'The question must be regarded, therefore, as still an open one. 



278. Multiplication by Binary subdivision takes place among the 

 Diatomacea on the same general plan as in the Desmidiacese, but with 

 some modifications incident to the pecularities of the structure of the 

 iormer group. The first stage consists in the elongation of the cell, and 

 the formation of a ' hoop ' adherent to each end-valve (| 274), so that the 

 two valves are separated by a band, which progressively increases in 

 breadth by addition to the free edges of the hoops, as is well seen in Fig. 

 167 A. In the newly formed cell e, the two valves are in immediate ap- 



Portions of Valve of Pleurosigma angulatum, as seen under a magnifying power of 2,000 dia- 

 meters, with central illumination; from a Photograph by Carl Gunther in the possession of the 

 Royal Microscopical Society. 



A. Normal hexagonal areolation; areolae bright circles, surrounded by dark hexagons. 



B. In upper part, areolae and their dark borders graduating from circular to elliptical; in lower 

 part, dark borders coalescing latterly, so as to give the appearance of continuous vertical 

 lineation. 



c. Areolae larger, brighter, and more elliptical; their dark bodies coalescing laterally, so as to 

 form very decided vertical lineation. 



D. Transition from hexagonal to triangular areolation, with three series of dark lines, one hori- 

 zontal and two oblique. 



position; in d, a band intervenes; in a, this band has become much 

 wider; and in b, the increase has gone-on until the original form of the 

 cell is completely changed. At the same time, the endochrome separates 

 into two halves; the nucleus also subdivides in the manner formerly 

 shown (Plate vin., fig. 1, G, H, i); and the primordial utricle folds-in, 

 -first forming a mere constriction, then an hour-glass contraction, and 



