246 



General Biology 



stigma, germinates to produce a reduced gametophyte and a pollen tube. 

 This pollen tube penetrates the soft stigma tissues and carries two sperm 

 toward the ovary cavity. As the pollen tube reaches the ovule, it enters 

 a tiny pore, called the micropyle, between the two integuments, and 

 then passes through the nucellus. The ovule is thus penetrated, and 

 one of the sperm unites with the egg and fertilizes it. 



The zygote now divides continually, and soon there is developed a 



'^^L^/^hjm 



<T A 



Fig. 149. Diagram of Life-cycle of an Angiosperm (Alisma 



Plantagoaquatica). 

 9, female gametophyte (embryo-sac) ; 8a and 9a, male gameto- 

 phyte (pollen-grain). (After J. H. Schaffner.) 



"tw*cUi^ 



wAJl, |v!auuu£ of 

 *X»ml»\, uvuuu. auwui 



Fig. ISO. 

 Development of the pea fruit from the pea flower. 

 Yung's Chart.) 



(After 



tiny stem with a little root at one end and two rudimentary leaves at 

 the other. 



The gametophyte has, in the meantime, resumed its development, 

 due to the union of the second sperm nucleus with the two polar nuclei, 

 to form the endosperm nucleus. This endosperm nucleus divides 

 rapidly, although the cell walls are much delayed in this development. 



