CYTOKINESIS AND THE CELL WALL 



167 



"budding" of yeast cells, and the abstriction of eonidia and basidiospores 

 may be regarded as special cases of cytokinesis by furrowing. 



The microspore quartets of most vascular plants fall into two classes 

 as regards the shape and arrangement of the spores. If no permanent 

 partition is formed after the first meiotic mitosis, the four spores are 

 delimited by partitions appearing simultaneously after the second mitosis. 

 Such spores, when first formed, commonly, 

 though not always, have the tetrahedral form. 

 If the first mitosis is followed immediately by 

 the formation of a permanent partition through 

 the equator of the sporocyte, each hemisphere 

 being divided by another partition after the 

 second mitosis, the quartet is said to be of the 

 "bilateral" type.'* 



Although the partitions delimiting the four 

 microspores in angiosperms are sometimes formed 

 by the cell-plate method, it has been shown^ that 

 furrows developing inward from the periphery 

 are chiefly responsible for cytokinesis here, at 

 least in cases of simultaneous division (quadripar- 

 tition) to form tetrahedral spores. In Nicotiana, 

 for example, the four microspore nuclei present 

 after the second mitosis all become connected by 

 achromatic "fibers" (Fig. 95). The two sets of 

 connecting fibers of the second mitosis may 

 persist, four new sets being added, or the two 

 may disappear, six sets being developed anew. 

 These fibers have nothing to do with the forma- 

 tion of the partitions; no cell-plates are devel- 

 oped. Furrows appear at the periphery and grow inward until they 

 meet at the center, dividing the protoplast simultaneously into four 

 spores. In Nelumbo the furrows are exceedingly narrow, appearing 

 much like cell-plates. As they grow inward they seem simply to cut 

 through any fibers which they may encounter (Farr). 



Meanwhile there develops within the original sporocyte wall a mass of 

 callose known as the "special wall." This increases in thickness and 

 follows the furrows inward, forming a sort of matrix in which the young 

 spores lie while their elaborate coats are being differentiated (p. 180). 



■* Although either the "simultaneous" or the "successive" mode of division may 

 tend strongly to predominate in certain groups of plants, there are so many exceptions 

 to rules and so much variation that the character is of very restricted taxonomic 

 value. See Tackholm and Soderberg (1917, 1918), Soderberg (1919), Palm (1920), 

 Suessenguth (1921), Stenar (1925), and Coulter and Chamberlain (1903). 



^C. H. Farr (1916, 1918, 1922a6), W. K. Farr (1920), Castetter (1925), Gates 

 (1925), W6ycicki (1932). 



Fig. 95. — Cytokinesis by 

 furrowing in the microsporo- 

 cyte of Nicotiana. {After 

 Farr, 1916.) 



