3 28 



EMBRYO OF CYCADS 



the ferns the gametospore divided into two cells which by further 

 division formed the sporophyte. The second departure now ap- 

 pears. The sporophyte or embryo ceases to grow and it is 

 admirably protected during its resting period by the integument 

 which has become differentiated into a stony inner coat and often 

 into a fleshy outer coat. This completed structure, consisting 

 of the modified integument, the embryo, and the gametophyte, 



Fig. 248. Development of the sporophyte or embryo of Cycas: A, sec- 

 tion of archegonium, showing a large number of cells lining the walls of 

 the germinating gametospore. This is the pro-embryo stage. B, later 

 stage in which the pro-embryo has developed a suspensor, s, which pushes 

 the embryo-forming cells, c, into the nourishing tissues of the gametophyte. 

 C, still later development, showing the greatly coiled suspensor and the 

 young sporophyte or embryo, e, with two cotyledons. D, section of a seed 

 of Zamia: i, integument which is fleshy without and stone-like within; 

 m, micropyle; g, gametophyte; c, dormant sporophyte or embryo. — A-C. 

 — After Treub. 



which has increased in size until it has absorbed all the tissues 

 of the sporangium, we call the seed (Fig. 248, D). At this 

 stage of development, the seed falls from the strobilus and may 

 remain in a dormant condition for years. 



The formation of the seed is the most characteristic feature 

 of the Spermatophyta and the most important advance that is 

 to be noted in the evolution of plant life. This variation has 

 given the seed plants an advantage over all other terrestrial 

 groups and made them the dominant plants upon the earth. Not 



