374 THE SPERMATOPHYTES 



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year for the female gametophyte to reach this stage of devel- 

 opment, when it occupies the greater part of the nucellus. In 

 the spring following the pollination of the cone, the endosperm 

 forms a group of several archegonia at its micropylar end. Each 

 archegonium (Fig. 300, D) consists of a much-reduced neck 

 region, generally composed of four cells, and the very large 

 egg which lies imbedded in the endosperm, whose cells form an 

 investment around it called the jacket. The egg is filled with 

 dense protoplasm and contains much food material supplied 

 through the cells of the jacket. 



This is the condition of the female gametophyte thirteen 

 months after pollination. At about this time the pollen tube 

 reaches the embryo sac and entering it passes between the 

 neck cells of an archegonium, where its tip fuses with the. egg 

 membrane. The contents at the end of the pollen tube are dis- 

 charged into the egg, including not only the two sperm nuclei, 

 but also the tube nucleus and that of the stalk cell. One of 

 the sperm nuclei moves towards the egg nucleus, which lies 

 near the center of the egg, and fusing with it completes the 

 act of fertilization. The other three nuclei break down and 

 soon disappear. 



355. The development of the embryo in the pine. Fertiliza- 

 tion takes place, as described above, a little more than a year 

 after pollination. The cone during this time has increased 

 greatly in size, but is generally hardly a third as large as the 

 mature seed-bearing cone. 



The fertilized egg soon begins to develop the pine embryo. 

 This is a complicated history, which cannot be described here in 

 detail. The embryo is however formed at the end of a structure 

 called the suspensor (Fig. 299, F, s), whose development carries 

 the embryo into the center of the endosperm, where it lies 

 in a favorable situation for its nourishment. The embryo 

 (Fig. 299, F, em] is straight, and the stem part is surrounded 

 by a circle of seed leaves called cotyledons. The pine seedling 

 is shown in Fig. 12. 



