feb., 1921] HARRIS AND OTHERS SEEDLINGS OF PHASEOLUS 



6 9 



jnderstandable the almost invariably twelve-bundled condition of the 

 lirst epicotyledonary internode. 



The structure of the normal seedling thus corresponds to the type found 

 oy one of the writers 6 to be characteristic of a large number of Angiosperm 

 families, in which the vascular supply to each cotyledon, consisting of two 

 strands, leaves but one gap in the vascular ring; and in which the foliage 

 *eaf is trilacunar. 



The Trimerous Seedling 



The seedling with three cotyledons and three primordial leaves is built on 

 different plan from the normal one in that it is prevailingly hexarch, six 



FIG. 7. Trimerous seedling. Transverse section through the root, showing its 

 ; hexarch condition. FIG. 8. Trimerous seedling. Transverse section through the base 

 : of the hypocotyl, showing the six primary double bundles. FIG. 9. Trimerous seed 

 ling. Transverse section through the mid-region of the hypocotyl, showing the nor 

 mal twelve-bundled condition. FIG. 10. Trimerous seedling. Transverse section just 

 below the cotyledonary node. The six bundles or bundle groups correspond in origin and 

 character to the four bundles of the dimerous seedling at this level. FIG. II. Trimerous 

 seedling. Transverse section through the cotyledonary node. Each group of three strands 

 bounded by a dotted line corresponds in origin and character to a similar group at this 

 level in the dimerous seedling. FIG. 12. Trimerous seedling. Transverse section through 

 the mid-region of the epicotyl, showing the eighteen bundles which have arisen by the 

 splitting of the nine original epicotyledonary bundles. The nine strands which are to go 

 off as traces to the three primordial leaves are solid black. 



6 Sinnott, E. W. Conservatism and variability in the seedling of dicotyledons. Amer. 

 Jour. Bot. 5: 120-130. 1918. 



