220 



BOTANY 



meiosis, 24 bivalents emerge from the second contraction ; they 

 undergo considerable condensation, until they acquire a more or 

 less definite form and the 24 pairs include an XY pair which are of 

 unequal size. At metaphase each of the bivalents is attached to 

 the spindle by one end and the two constituent elements move 

 apart. The X-chromosome is considerably larger than the Y- 

 member, and they pass to the opposite poles of the spindle. 

 The second division is equational, so that the pollen grains are of 

 two kinds, two with nuclei containing an X-chromosome and two 



:<Wm 



iyi^^-^X^'-v 



Up 



■'/?Ssv 



^<t 





3 



Fig. 79. — Microsporogenesis in Elodea canadensis. (1) Diakinesis, showing 

 unequal chromosome pair, fm (XY). (2) Beginning of anaphase, X 

 and Y disjoining ( / and rn). (3) Metaphase, showing X and Y (/ and //*). 

 (After Santos, Bot. Gaz.) 



with the Y-chromosome. In the female plants, the nuclei contain 

 the elements XX and in the male plants contain an XY pair. 



Correns showed in Rumex acetosa, that the staminate plant was 

 dispone and that there was a very considerable difference in the 

 rate at which the pollen tubes carrying male and female deter- 

 mining gametes grow down the styles. Later the matter was 

 carried further by Kihara and Ono, who showed that in the 

 staminate plant the somatic number of chromosomes is apparently 

 15. In the case of the microsporocytes there are six gemini and a 

 triple group composed as follows : a large chromosome (M-chromo- 

 some) and a smaller member attached one to either end (m^ and mg). 



