172 



Colchicine 



that action during division leading up to niciosis creates octoploid 

 or tetraploid pollen mother cellsJ'^ In contrast, activity dm ing meiotic 

 divisions I and II creates tetraploid monads, and activity at division 

 II only, diploid monads. Monadal formation is a special feature of 

 the c-meiosis. The monads replace the usual tetrads of microspores 

 forming at the close of a meiosis.-^' ^*^- ^-~- "^^ 



Since archesporial divisions become regular c -mitoses, these are not 

 described in great detail here, except to say that one c-mitosis in this 



MITOSIS 



B 



Z 



NORMAL 



COLCHICINE 



C. 



MEIOSIS 



A N 



NORMAL 



c 



Fig. 4.4 — Comparison of a c-meiosis and c-mitosis. The stage reached when colchicine 

 becomes effective determines the action in meiosis. (After Levan) 



tissue gives rise to tetra})loid j^ollen mother cells, and that two c- 

 mitoses bring about the octoploid condition. Beyond this degree of 

 ]«)lyjjloidy the meiotic processes are so upset that no finther action 

 of colchicine can be obtained at meiosis. The premeiotic stages of 

 Allium ccniiiinn with diploid, tetraploid, and octoploid numbers 7, 

 14, and 28 pairs, respectively, were observed and followed up to the 

 first meiosis.'^'' Already at tetraploid stages, the polarities of meiotic 

 spindles were irregular. The multii>le spindle aspects dining re- 

 covery from a c-mitosis were noticed at meiosis if the previous c-mitotic 

 cycles of archesporial cells caused polyploidy. 



Pairing of homologous chromosomes and chiasmatal formation 

 formed during prophase are decisive functions before a regidar meiosis 



