THE EMBRYO AND SEEDLING 137 



the leaflets. After this age the new leaves appear with 

 leaflets throughout nearly the entire length of the mid- 

 rib, the lowest leaflets being merely spines, above which 

 the leaflets graduafly increase in length (Fig. 77). 



Ferns are characterized by their circinate "verna- 

 tion," the term meaning that in the bud the tip of the 

 leaf is rolled in so that it looks like crozier. Many 

 cycads, like Cycas, show this type of vernation, which was 

 characteristic of their remote Paleozoic ancestry; but 

 in others, like Dioon, the leaves lie perfectly straight in 

 the bud. 



The anatomy of the seedhng has been studied chiefly 

 by Thiessen and by Sister Helen Angela, both of whom 

 have traced the development of the vascular system 

 from its first appearance up to a stage in which the 

 seedling has several leaves. Here again there are linger- 

 ing structures retained from the remote fern ancestry. 

 The peculiar "girdling" of the numerous bundles which 

 supply the leaves is present even in the first leaf of the 

 seedling, but the bundles start straight, the girdhng 

 being due to the great radial growth of the leaf base. 



It would be interesting to know the anatomy of the 

 seedlings of the Paleozoic Cycadofilicales and the 

 Mesozoic Bennettitales, but this information is not yet 

 available. 



The vascular anatomy of both the seedling and the 

 adult plant deserves more attention than we have given 

 it, but the extensive technical vocabulary which seems 

 necessary in dealing with this subject has led us to omit 

 much of this important source of evidence. 



The seedling develops gradually into the adult plant, 

 the stage of the life-history with which we started. 



