THE ANGIOSPERMAE 



1377 



Equator. Division may be simultaneous throughout the pollen loculus, 

 particularly where the grains are united into massulae or pollinia, where the 







Fig. 1280. — Pollinium of Gymmideuia conopsea, showing 

 synchronization of nuclear divisions in all the compo- 

 nent grains. (After Barber.) 



grains are not separated by cutinized walls (Fig. 1280), but frequently 

 there is no accurate synchronization, though the differences in timing are 

 not great. 



Early in the development of the grain a vacuole appears in it, towards one 

 end, which pushes the nucleus and the bulk of the cytoplasm towards the 

 other end, but in some types [Tradescantia) a second vacuole appears at the 

 cytoplasmic end and then nucleus and cytoplasm are held between the two 

 vacuoles (Fig. 1281). 



When mitosis occurs the angle of the nuclear spindle to the wall of the 

 grain is constant in a species and even in whole genera. The position in 

 which the generative cell is formed is determined by the position of the 

 nuclear spindle, but in the majority of species it is cut off towards the pole 



