122 DICOTYLEDONEUS ANGIOSPERMS 



That dicotyledons are extremely variable will be better 

 understood by a study of Part II which deals largely with 

 dicotyledenous plants. 



SUMMARY OF DICOTYLEDONS 



Gametophyte. The male and female gametophytes have 

 structures which are essentially like those of the monocoty- 

 ledons. 



Male Gametophyte. The male gametophyte is formed by 

 the growth of the pollen grain. It consists of two cells only, 

 the generative cell and the tube cell. The tube cell grows 

 down through the tissues of the style to the ovule and to the 

 embryo sac where it empties the two sperm cells resulting 

 from the divisions of generative cells. 



Female Gametophyte. The megaspore, contained in the 

 ovule and enclosed by the ovary, divides to form the female 

 gametophyte or embryo sac which develops the egg cell. 



Double fertilization occurs, as in the monocotyledons; one 

 sperm cell unites with an endosperm cell. The fused cell 

 divides to form the endosperm. The other sperm cell unites 

 with the female or egg cell to form the embryo sporophyte 

 which is characterized by having two opposite cotyledons 

 and a terminal stem bud or plumule, a fact which gives the 

 name dicotyledons to the group. 



Sporophyte. The sporophytes of the dicotyledons vary 

 from minute plants to giant trees. All have roots, stems, 

 leaves and flowers. 



Roots. The roots which are branches of the primary root 

 arise endogenously from the procambium. The tip of the 

 root ends in a root-cap which protects the formative cells; 

 these divide to increase the length of the roots. The pilif- 

 erous or root-hair layer, which functions as absorbing tissue, 



