142 GYMNOSPERMS 



In Dioon cdulc the egg often reaches a length of 5 mm., and the num- 

 ber of free nuclei is about i ,000 — theoretically i ,024. In Zamia Jlori- 

 dana, with an egg about 3 mm. in length, the number is quite regu- 

 larly 256. In BoK'cnia scrrulata, Lawson^^z reports 64, the lowest 

 number ever found in a cycad. In other gymnospcrms, with much 

 smaller eggs, the number comes down to 4, and in Sequoia a wall is 

 formed at the first mitosis. 



In the homosporous ferns there is no free nuclear period, and none 

 in living heterosporous ferns, although it is probable that there was 



Fig. 153. — Stangeria paradoxa: simullancous division of nuclei in lower part of em- 

 bryo; X140. — After Chamberlain."' 



a free nuclear period in extinct heterosporous ferns, and, in our 

 opinion, there was an extensive free nuclear period in some of the 

 Cycadofilicales. A free nuclear period arose as a consequence of the 

 enlarging eggs. The mass of protoplasm became so large that the 

 early mitotic figures could not segment it. 



In Cycas and Stangeria the nuclei at the base of the embryo under- 

 go a vigorous simultaneous division after the nuclei of the rest of the 

 embryo have ceased to divide (fig. 153). The suspensor, and all the 

 rest of the embryo below it, come from this second period of simul- 

 taneous division. 



Formation of cell walls in embryo. — After the nuclei have increased 

 in number until there is a comparatively small amount of proto- 



