i62 THE LIVING CYCADS 



correlated with the development of the tuberous, sub- 

 terranean habit so that, in this respect, genera like Cycas 

 would represent the beginning of the line, and Slangeria 

 the most extreme reduction. 



The course of the leaf bundles in the cortex is direct 

 in the Cycadofihcales and Bennettitales, while most of 

 the cycads are characterized by the girdhng of these 

 bundles. Consequently cycads like some species of 

 Macrozamia, which show a direct course of the bundle 

 in the adult plant, and Bowenia, which shows a similar 

 arrangement in the seedhng, are nearer the fern condi- 

 tion in this respect. In the microscopic structure of the 

 wood Stangeria, with its scalariform tracheids, seems to 

 present the least divergence from the fern habit. 



THE ROOT 



Comparatively little is known about the roots of the 

 living cycads, and practically nothing is known about 

 the development of the roots of their Mesozoic and 

 Paleozoic predecessors. 



The building up of a root from the segments of a 

 single apical cell is characteristic of ferns, but no such 

 cell has yet been described in the cycads, the root 

 developing from a group of cells, as in the highest 

 flowering plants. We should not be surprised to find 

 the single apical cell in the Cycadofihcales, but at present 

 we could only guess at conditions in Paleozoic, Mesozoic, 

 and even most of the living members of the phylum. 

 Growth by the single apical cell is doubtless the primitive 

 method, and growth by a group of cells has been derived 

 from it, but in this character the cycads seem to present 

 such a uniformity that no trend is distinguishable. 



