OF ARTS AND SCIENCES. 117 



mass of cells which forms the cortical layer or wall of the frond. The 

 set of cells derived from each apical cell is easily distinguished within 

 an area of forty micro-millimeters from the apex, because the cells hav- 

 ing a common origin are separated from each other within this region 

 by thinner cell walls than those separating cells of different origin, as 

 shown in the figure. Below this area the cell walls become of equal 

 thickness Cell division takes place chiefly within a short distance 

 from the apex ; below that, growth takes place principally by enlarge- 

 ment of the cells. "Within the area of division the cells are filled with 

 protoplasm and have very evident nuclei. The nuclei in the cortical 

 layer are usually in the lower half of the cell. Below this area around 

 the apex the cells contain large vacuoles and the nuclei become much 

 less prominent, while the protoplasm becomes more coarsely granular. 

 The inner cells above mentioned as the result of the first division below 

 the apex do not divide at right angles to the surface of the plant ; at 

 least, if they do, it is a much less frequent process than in outer cells ; 

 but many of them, perhaps all, do divide once or twice by partitions 

 parallel to the surface (Fig. 8). As the cortical layer grows, in- 

 creasing the length and diameter of that part of the frond, these 

 inner cells merely elongate, while they become separated laterally, 

 and so form the longitudinal filaments; as described by Wille for 

 Lomentaria. 



By means of the division parallel to the surface just mentioned 

 are produced the "bulb-cells,'' and the connections behind them with 

 the cortical layers (Fig. 8). The "bulb-cells" attain their adult 

 size very soon after their formation. At intervals of three cells or 

 more, on certain filaments, division is carried further, until the pro- 

 cesses pushed out from them in this way meet in the middle of the 

 cavity (Fig. 9). The spaces between these processes are filled by 

 similar ones from the other filaments, and in this way the diaphragm 

 is formed. At first the diaphragm shows very plainly its origin in 

 branches from the filaments. The cells are rounded, contain prom- 

 inent nuclei, and in short look just like the young filament cells. 

 Then the cells of each of the component branches are separated from 

 each other by thinner walls than those which separate them from the 

 other cells. This formation of the diaphragm occurs about thirty micro- 

 millimeters from the surface of the apex. The young diaphragm keeps 

 pace with the rapid growth of the adjacent parts of the plant, and thus 

 preserves its continuity, by further cell division and by increase in the 

 size of the cells. These cells finally become polygonal from mutual 

 pressure, the cell walls become equally thick on all sides, and it be- 



