120 



of small granules appears in the middle of the space occupied by 

 the connectnig threads, and careful examination shows them to be 

 knot-like thickenings of the latter (Fig. ii C.) This struc- 

 ture is the *' cell plate/' and usually is the beginning of a division 

 wall (PL CII. Figs. 12-14). In this case {Allhtm) new connecting 

 threads are formed, running to the circumference of the cell plate, 

 thus increasing its size until finally it reaches completely across 

 the cell (Fig. 13). The separate microsomes of which it is com- 

 posed now coalesce and form a complete membrane, that divides 

 the mother cell into two. In Podophylliun and most other dico- 

 tyledons, the primary cell-plate disappears without forming a con- 

 jtinuous membrane, though the indication of a division is often 

 evident even after the connecting threads are no longer evident. 

 (PL CIILFig. 13). 



Each of the daughter-nuclei now goes through the same pro- 

 cess described for the primary nucleus, but when the ripe spores 

 are arranged tetrahedrally, as in Podophyllmn^ the four nuclei 

 resulting from the final division arrange themselves about the 

 periphery of the cell at equal distances from each other, and con- 

 necting threads are formed between them in all directions (PL CIII. 

 Fig. 17). A cell-plate is now formed between each pair of nu- 

 clei, extending to the center of the cell, thus forming four tctra- 

 hcdral cells — the young pollen-spores. The division wall is formed 

 in part also by a ridge which projects inward from the wall of 

 the mother cell (Fig. 19). The division walls become finally very 

 thick, so that the young pollen spores appear to be embedded in 

 a thick colorless matrix. This thick wall is finally absorbed, set- 

 ting free the spores and these soon acquire their characteristic 



covermg. 



In A/Hum and most monocotyledons, the spores are of the 

 " bilateral " form, and in these a dividing membrane is formed 

 after each nuclear division. The division of the daughter-nuclei 

 in these frequently takes place in planes at right angles to each 

 other (PI. CII. Figs. 10, 14). 



Direct nuclear divisions (/. e., constriction of the nucleus with- 

 out any further change) is known In a few cases, but always takes 

 place in the older cells. The older parenchyma cell of the stem 

 of Tradcscantia Virginica and the internodal cells of Chara and 



