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70 BOTANICAL GAZETTE [December 



the protoplast. At this stage in its development the kinoplasm disappears 

 from two sides of the phragmoplast, leaving two entirely separate aggregations 

 of kinoplasmic fibrillae. These rod-shaped masses of kinoplasm (kinoplas- 

 masomes) move in opposite directions, thereby extending the cell plate, until 

 it eventually reaches the two ends of the protoplast. This type of cytokinesis 

 is not confined to the cambium, but occurs in other somatic tissues, in elon- 

 gated or much flattened cells whose planes of division have one long and one 

 short dimension. 



A comparative study of cytokinesis in different somatic tissues and in 

 cells of different shapes and sizes indicates that the various types of cell plate 

 formation, described by Treub, Strasburger, Schurhoff, and the writer, 

 are but different phases or stages of a single fundamental type of cytokinesis. 

 The particular expressions of the phenomenon which may occur in a given 

 cell are dependent upon the dimensions of the latter, its plane of division, and 

 the size and location of the nucleus. Thus, in very small isodiametric cells, 

 having a large centrally located nucleus, the cell plate quickly intersects the 

 walls of the cell, without any extensive lateral growth of the phragmoplast. 

 In larger elements there is sufficient room for the phragmoplast to attain the 

 "halo" stage before the cell plate intersects the sides of the protoplast. 



The striking similarity between Beer and Arber's figures of sections of 

 "phragmospheres" and polar views of normal ring-shaped phragmoplasts, 

 such as occur in the cambium and other tissues of the higher plants, raises 

 the question whether the phenomena encountered by these investigators are 

 not actually stages in the division of parenchymatous cells. Although they 

 state that the whole phragmoplast with its associated cytoplasm becomes 

 transformed into a hollow sphere which incloses the daughter nuclei, and that 

 the cell plate is resorbed without forming a membrane, they present no con- 

 clusive evidence in favor of such a hypothesis. For example, they give no 

 critical figures or detailed descriptions to elucidate various stages in the for- 

 mation of the phragmospheres or the resorption of the cell plate. They state 

 that the phenomena occur in cells which are not forming division membranes, 

 yet the binucleate phase is admitted to reach its most characteristic expression 

 in young active tissue, just previous to the maximum period of growth under- 

 gone by the region of the stem, leaf, or root in which it occurs. Of course,, 

 the fact that individual cells are increasing in diameter does not indicate 

 necessarily that they have ceased to divide in a given plane. Although the 

 material that I have studied is extremely favorable, owing to the large size 

 of the cells, nuclei, and division figures and their symmetrical arrangement, I 

 have frequently found it difficult to determine, in a given plane of section, 

 whether cells contained more than one nucleus each. This is due to the fact 

 that nuclei which appear to lie within a single protoplast, that is in the same 

 focal plane, subsequently are found to be separated by thin, recently formed 

 membranes or cell plates. Beer and Arber's conclusions seem to have 

 been drawn largely from the study of transverse sections of stems and roots. 





