466 



SPERMATOPHYTES (SEED PLANTS) 



connection with double fertilization it should be noted that the 

 endosperm nucleus contains the contents of three nuclei, since 

 it is a product of a triple fusion, involving a sperm and the two 

 polar nuclei. 



Embryo. — The first cells produced by the division of the fer- 

 tilized egg form a filament which pushes down into the embryo 

 sac. This filament is called the proembryo. The terminal cell 

 of the proembryo develops the embryo, while the remainder of 

 the filament remains as a stalk called suspensor. After the termi- 



FiG. 412. — Development of the embryo and endosperm in the Shepherd's 

 Purse. A, section through ovule with embryo and endosperm in early stage 

 of development, showing the proembryo which- consists of the suspensor (6) 

 and the terminal three-celled embryo (a), and also showing the endosperm (c) 

 as a chain of free nuclei around the wall of the embryo sac. B, the same as 

 A, excepting that the proembryo and endosperm are more developed. C, 

 section through a mature seed showing the seed coat (s), and the mature 

 embryo with cotyledons at h, plumule at p, hypocotyl at e, and radicle at d. 



nal cell divides a number of times, the parts of the embryo begin 

 to be differentiated. In Dicotyledons two lobes appear at the 

 end farthest from the micropyle and these become the two coty- 

 ledons characteristic of dicotyledonous Angiosperms. Between 

 the cotyledons the plumule is formed, while the axis of the embryo 

 below the cotyledons is differentiated into the hypocotyl, which 

 is the main part of the axis, and the radicle at its lower end 



{Fig. m). 



The embryos of monocotyledonous Angiosperms have a radicle, 

 hypocotyl, plumule, but only one cotyledon. They also differ 



