346 



INTRODUCTION TO CYTOLOGY 



tion (Randolph, 1932). It has been observed in roots of Spinacia that 

 the chromosome halves may separate during the prophase and undergo a 

 second splitting so that each telophase nucleus receives the tetraploid 

 number (24) rather than the diploid (12). Repetition of the process may- 

 give octoploid nuclei." Sometimes diploid plants show tetraploid sporo- 

 cytes undergoing meiosis along with the normal diploid ones, which sug- 

 gests the occurrence of non-division in the development of the anther. A 

 similar condition has been seen in the spermatocytes of Euschistus.-^ In 

 Zea the occasional appearance of triploid plants strongly suggests that 

 functional diploid spores and gametes may thus arise and lead to hetero- 

 ploidy. The origin of tetraploidy through a fusion of two such gametes 

 would depend on the occurrence of the aberration in the lineage of both 

 sperms and eggs. In a monoecious plant a single aberration of this type 



ABODE 



Fig. 197. — " Semiheterotypic " mitosis, with formation of "restitution nucleus" in 

 Hieracium. A, B, prophase; C, irregular metaphase; D, irregular anaphase. E, polar 

 view of metaphase of division of restitution nucleus formed when all of the chromosomes 

 in the preceding anaphase (not all shown in D) reorganize as one nucleus instead of two. 

 {After Rosenberg, 1927a.) 



occurring before the differentiation of the carpellate and staminate por- 

 tions might result in the production of diploid gametes of both sexes, 

 whereas in a dioecious plant or in a monoecious one after the differentia- 

 tion of the carpels and stamens, two similar aberrations would be required 

 for the production of tetraploidy in the immediate offspring. 



Another aberration in sporogenesis is the formation of a "restitution 

 nucleus" (Fig. 197). Usually after deficient synapsis the chromosomes 

 are scattered along the first meiotic spindle, and before they reach the 

 poles a single large nucleus is formed, enclosing all of them. This nucleus 

 and the sporocyte then divide once, giving two large microspores, each 

 with the unreduced number of chromosomes. ^^ In several instances 

 nuclei formed by the meiotic mitoses have been seen fusing in the sporo- 



^^ De Litardiere (1923c). Other probable cases of non-division: Cannabis (de 

 Litardiere, 1924; Langlet, 19276) and Lycopersicum (Lesley, 1925). 



2« Randolph (1928) and McClintock (1929a) on Zea; Bowen (1922/) on Euschistus; 

 Fukushima (1921) on Brassica; Geerts (1909) on megasporocyte in Oenothera. 



29 Rosenberg (1917, 19266, 1927a6) on Hieracium^ Kuwada (1928) on Balanophora, 

 Tischler (1928) on Ribes, Kagawa (19296) on Triticum, Buxton and Newton (1928) 

 on Digitalis. 



