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INTRODUCTION TO CYTOLOGY 



later in this cleft. In living stamen hairs of Tradescantia, W. Becker 

 (1932) observes that the cell-plate material appears first in the form 

 of minute droplets which then unite to form a continuous plate (Fig. 99). 

 This may be a more adequate description of what was formerly inter- 

 preted as "swellings of the spindle fibers." On the basis of such observa- 

 tions the supposed "splitting of the cell-plate to form the plasma 

 membranes" of the two protoplasts could be regarded as merely the 

 widening of the fluid layer as its substance increases in amount and receives 

 deposits of the primary wall materials. The plasma membranes would 

 be the protoplasmic surfaces lying against the fluid layer. The origin of 

 the fluid which collects thus at the equator to form the cell-plate is 

 unknown, but it has been claimed that a stainable substance migrates to 



Fig. 99. — Formation of cell-plate by coalescence of droplets in stamen hair of Trade- 

 scantia. The changes shown occupied 27 minutes. {After W. A. Becker, 1932c.) 



this region from the two young nuclei. ^° The observations of Becker 

 point rather to the conclusion that there is a local dissociation of two 

 protoplasmic phases, one of these forming the cell-plate while the other 

 remains as the plasma membranes. Such a dissociation ("Entmis- 

 chung"), often reversible, of more and less fluid phases has been induced 

 in the living protoplasm of Didymium and Tradescantia by Belaf (1930). 

 There are many facts which suggest that the processes of cytokinesis 

 by furrowing, by vacuoles, by membranes, and by cell-plates are not 

 wholly distinct from one another. The formation of the fluid cell-plate 

 from the center outward suggests the cleaving action of vacuoles in the 

 sporangium of Phycomyces; whereas, when spores begin to separate at 

 the periphery after cytokinesis by cell-plates, the appearance is that of 

 furrowing. It may be helpful to think of cytokinesis by cell-plates in 

 vascular plants as a process whose peculiarities are due in large measure 

 to its close association with the mechanism of karyokinesis. 



1" Dembowski and Ziegenspeck (1929), Dannehl and Ziegenspeck (1929). 



