36o 



THE EMBRYO OF CYCADS 



rudimentary root, stem and two cotyledons (Fig. 248, C). In 

 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 middle layer and into 

 a fleshy inner and outer layer. This completed structure, con- 



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, e, 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; w, micropyle; 

 g, gametophyte; e, dormant sporophyte or embryo. — A-C. — After Treub. 



sisting of the modified integument, the embryo, and the gameto- 

 phyte, 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 vaiiation has 

 given the seed plants an advantage over all other terrestrial 

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



