THE ANGIOSPERMAE 



1379 



to cut ofF the generative nucleus in a small, lenticular cell against the wall 

 of the grain. From this position it normally becomes free after a short time, 

 and is then seen as a biconvex cell, surrounded by the cytoplasm of the 

 vegetative cell. It is always smaller than the vegetative cell but its shape is 

 variable, from spherical in some species to a long cigar-shape, the ends of 

 which may almost encircle the vegetative nucleus. The grain is now mature; 

 all vacuoles disappear and starch grains or oil drops accumulate (Fig. 1282). 





Fig. 1282. — Liliiini auratiun. Mature pollen grains shcnving vegetative nucleus and generative 

 nucleus, the latter surrounded by hyaline cytoplasm. Note the pila of the sexine. 



The vegetative nucleus does not normally divide again, though abnor- 

 mal cases of from one to several divisions have been recorded. The genera- 

 tive nucleus may divide either before the pollen germinates or in the pollen 

 tube. The latter is the commoner case but precocious division may occur 

 even before the pollen is shed from the anther. Sometimes it occurs on the 

 stigma, before the pollen tube begins to grow. At the other extreme, a case 

 has been observed in Euphorbia in which division of the generative cell was 

 postponed until the pollen tube had actually entered the embryo sac. 



When division takes place in the pollen grain the nuclear division seems 

 to follow the lines of a normal mitosis, a spindle being formed and the 

 generative cell also dividing, either by a cross-wall or by constriction. Thus 

 a three-celled grain results. 



The process of division in the pollen tube will be described later when 

 we speak of fertilization (p. 1442). 



