No. 450.] STUDIES ON THE PLANT CELL. 



459 



the neighboring cell walls, fusing with the outer plasma mem- 

 brane. There are certain mitoses, as in some spore mother cells 

 and in the embryo sac (see Section III) where the cell plates 

 are absorbed into the cytoplasm leaving the original cell with 

 two or more nuclei and without partition walls. It is uncertain 

 whether the edge of the plate is ever extended by the develop- 

 ment of additional peripheral fibrillae (Timberlake, : 00, p. 161) 

 from the daughter nuclei. 



Cell division is accomplished by the splitting of the cell plate 

 (Strasburger, '98) into two plasma membranes. The division 

 generally begins in the center and the cleft progresses towards 

 the periphery until it reaches the cell wall. During the process 

 the thickened rod shaped portions of the spindle fibers are pulled 

 apart. There are thus left two kinoplasmic membranes opposite 

 one another and continuous with the outer plasma membrane 

 surrounding the daughter cells. The cause of this cleavage is 

 not apparent, but there are reasons for believing that the split 

 is essentially a thin vacuole which, starting near the center, cuts 

 its way through the cell plate to the periphery after a manner 

 very similar to the behavior of vacuoles during the cleavage of 

 the Plasmodium and in the sporangia of certain moulds. And 

 there may be shown in this activity a relationship of cleavage by 

 cell plate to some of the events of cleavage by constriction. 

 After division is complete there follows the formation of a cell 

 wall between the two cell surfaces after the method usual to 

 plasma membranes. 



The new cell wall generally begins in the oldest portion of the 

 cell plate where the cleft first appeared and is gradually built 

 out peripherally until it reaches the side walls. The first indica- 

 tion of the wall is the appearance in the cleft of a stainable 

 carbohydrate substance which resembles the material that was 

 primarily present between the fibers of the central spindle and 

 which disappears with the formation of the cell plate. This 

 material is probably the basis of the first deposits on the surface 

 of the two plasma membranes, but the nature of the final sub- 

 stance is exceedingly various. A cell wall may be formed that 

 is homogeneous throughout but often the thickened wall presents 

 three regions, two layers of a cellulose basis formed by the 



