SEED 



457 



middle of June of the second season and fertilization soon follows. 

 In addition to its simplicity the notable features of the male 

 gametophyte are that the sperms have no cilia and that they are 

 conducted to the archegonia by the pollen tube. 



Seed. — The fertilized egg at first forms tiers of cells, which 

 constitute a long filament, called a suspensor, at the end of which 

 the embryo develops deeply imbedded in the nutritive tissue of 

 the female gametophyte. When mature the embryo is still 

 surrounded by much gametophytic tissue called endosperm. 



While the embryo or the young sporophyte is developing, the 

 ovule and the entire cone continue to enlarge. The integument 

 is transformed into a seed coat, and when mature the seed sepa- 



Iree 



Y»ee 



Fig. 406. — Diagram of the life cycle of the Pine. Starting with the tree 

 at the left, the two kinds of strobili are shown at a and b, the two kinds of 

 sporophylls and their sporangia at c and d, the two kinds of spores at e a,nd /, 

 the gametophytes at g, the mature seed at h, from the embryo of which a 

 new tree develops. 



rates from the ovulate scale with a long membraneous wing, 

 which enables the seed to float in the air {Fig. 405.) Pine 

 seeds, although usually smaller, are similar in general structure 

 to the seeds of Cycads. They contain a female gametophyte 

 bearing a young sporophyte and a protective covering composed 

 of the integument and the nucellus, the latter persisting as a 

 membrane about the gametophyte or endosperm. 



The scales of the ovulate strobilus continue their development 

 until the seeds are mature and remain tightly closed so that the 

 seeds are well protected. After the cone is mature, the scales dry 



