126 



THE LIVING CYCADS 



the embryo, although the plant is built up exclusively 

 from the lower portion, the larger upper region serv- 

 ing only as food material. 



With the ninth or tenth divi- 

 sion, in all cases which we have 

 observed, the period of free 

 nuclear divisions without the 

 formation of cell walls comes to 

 a close. Some conclusions with 

 regard to the origin, cause, and 

 evolution of the free nuclear 

 period will be found on page 155. 

 The first permanent walls at 

 the base of the embryo are 

 formed simultaneously, since 

 they result from a simultaneous 

 division of the nuclei (Fig. 67). 

 Subsequent divisions are not 

 simultaneous, doubtless because 

 each nucleus is now inclosed in 

 its own cell, and the nuclei are 

 no longer exposed to the uniform 

 conditions which prevailed when 

 they were in one common mass 

 of protoplasm. 



Almost immediately after per- 

 manent walls begin to appear the 

 cells become differentiated into three regions: the upper, 

 in contact with the large mass of protoplasm and nutri- 

 tive materials, consisting of cells which become more 

 or less haustorial in fuiuiii)n; the lower, consisting 

 of smaller cells with rich protoplasmic contents; and 



Fig. 66. — Siaiigcria 

 paradoxa: free nuclear 

 stage in the development 

 of the embryo, showing 

 evanescent segmentation 

 at the top. Highly 

 magnified. 



