No. 450 ] STUDIES ON THE PLANT CELL. 



4.S7 



Plasmodium or sporangium where the cleavage planes are foi 

 without regard to the time of nuclear division or the positif 



2. Cleavage by Cell Plates. 



Cleavage of the protoplasm by means of the cell plate is 

 almost universal in cell division, of plants above the thallophytes. 

 It is one of the peculiarities of plant cells, having been found in 

 comparatively few animals and there represented rather imper- 

 fectly by the so-called mid-body. The general events of the 

 process have been known since Treubs' studies of nS/S, and 

 were clearly described by Strasburger in 1880. Timberlake, 

 :oo, in a recent paper gives an historical review of the subject. 



When, after the metaphase of mitosis, the two sets of daugh- 

 ter chromosomes separate from one another there is left between 

 them the spindle, made up of the central fibers. The first 

 appearance of the cell plate is a line of granules in the equatorial 

 region of this spmdle where the nuclear plate formerly lay. 

 But several important events proceed this condition. The con- 

 necting central fibers begin to thicken, first near the daughter 

 nuclei, and then gradually towards the equatorial region of the 

 spindle. The number of fibers may increase greatly, probably 

 by the separation of bundles of fibrillae composing the spindle 

 into independent elements (Timberlake, : 00, p. 94)- But there 

 is evidence that new fibrilte are sometimes formed from the 

 vicinity of the daughter nuclei, some of which may enter the 

 spindle and cooperate with the connecting fibers. In certain 

 forms {e. g. Allium) there is an accumulation of a stainable sub- 

 stance between the connecting fibers in the equatorial region of 

 the spindle. The reaction of this substance to stains indicates 

 a carbohydrate composition. 



The cell plate really begins with the thickening ot the con- 

 necting fibers in the equatorial plane of the spindle. In some 

 forms these thickenings are elongated bodies, in others mere 

 granules. The earlier writers (Treub, '78, Zacharias, '88) did 

 not believe that they came from the spindle fibers, but there 

 seems to be now no doubt of their origin from these elements, 



