THE EVOLUTION OF STRUCTURES i6i 



free nuclear period. In the embryogeny, as we have 

 just stated, the series begins with small eggs in which 

 a wall follows the first nuclear division, and the series 

 ends with the same condition; but in the gametophyte, 

 while the series begins with a condition in which a wall 

 follows the first nuclear division, then shows a gradu- 

 ally increasing free nuclear period, and then a decline of 

 the free nuclear period, the decline never quite reaches 

 the starting-point of the ascending series, for even in the 

 highest flowering plants with exceedingly small gameto- 

 phytes there is no formation of cell walls until a sec- 

 ondary development resulting from fertilization is 

 initiated. 



THE LEAF 



The fern type of leaf has been maintained with 

 remarkable persistency throughout the entire phylum, 

 from its earliest appearance in the Paleozoic, through 

 the Mesozoic, and in all the living genera. Some cycads 

 still show the circinate, or coiled, arrangement of the 

 leaf as it unfolds from the bud, a striking fern character. 

 The forked veins, so characteristic of ferns, appear in all 

 the living genera except Slangeria, and this exception is 

 one which also occurs in ferns, so that the leaf of Stangeria 

 may be as conservative as any of the others. On the 

 basis of the leaf we should not attempt to decide whether 

 one cycad is more advanced than another. 



THE STEM 



The stem affords a few characters which may indicate 

 the trend of development. The columnar stem, covered 

 by an armor of leaf bases, we should regard as the primi- 

 tive type, the disappearance of armor being more or less 



