CHAPTER XII 

 CYTOKINESIS AND THE CELL WALL 



The division of the cytosome, or extra-nuclear portion of the proto- 

 plast, is known as cytokinesis. It may occur more or less independently 

 of karyokinesis (mitotic nuclear division), or the two processes may be 

 very intimately associated. Cytokinesis is accomplished in a variety of 

 ways in growing and differentiating masses of protoplasm. The prin- 

 cipal types are included in the following description.'^ 



Furrowing in Plants. — The cleavage of plasmodial masses is most 

 commonly brought about by the formation of furrows, with or without 

 the cooperation of vacuoles. It is well illustrated in the sporangia of 

 myxomycetes and certain fungi. ^ In Fuligo, cleavage furrows begin to 

 develop at the peripheral membrane of the young sporangium and 

 gradually extend inward, cutting out multinucleate blocks which are 

 subdivided by further furrowing into uninucleate spores. In Didymium 

 the spores are delimited in a similar way by furrows which begin to 

 develop both at the periphery and along the young capillitium filaments 

 in the midst of the protoplasm. In Rhizopus the furrows develop from 

 the peripheral membrane (Fig. 92, A) and from the columella. In the 

 sporangia of Achlya, Saprolegnia, and Olpidiopsis the furrows start from 

 a large central vacuole. In Phycomyces, vacuoles appear in the proto- 

 plasm, become stellate in form, and cut out spore masses with from one 

 to about twelve nuclei each (Fig. 92, B). Such vacuoles function together 

 with peripheral furrows in Piloholus and Circinella. In the sporangia of 

 the phycomycetes the columella is separated from the rest of the sporan- 

 gium by a dome-shaped layer of vacuoles which coalesce and form a 

 continuous partition between the two regions. Such cleavage is inter- 

 preted by Scarth (1927) as an extension and swelling of kinoplasm present 

 at the protoplasmic surfaces. 



Similar forms of cytokinesis are found in certain algae. ^ In Hydro- 

 dictyon, for example, the multinucleate protoplasm, lining the wall of the 



1 A classification of types of cytokinesis, with examples, is given by Yamaha 

 (1926a). 



2 Harper (1899, 1900a, 1914) on Synchytrium, Piloholus, Sporodinia, Fuligo, and 

 Didymium; D. B. Swingle (1903) and Moreau (1913) on Rhizopus and Phycomyces; 

 Rothert (1892) and Schwarze (1922) on Saprolegnia, Circinella, and Achlya; B. M. 

 Davis (1903) on Saprolegnia; Rytz (1907) on Synchytrium. 



3 Klebs (1891) and Timberlake (1902) on Hrjdrodictyon, Yamanouchi (1906) on 

 Polysiphonia, Brand (1908) on Cladophora. For an extensive account of the algae, see 

 Oltmanns (1922-1923). 



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