Feb., I92ij HARRIS AND OTHERS — SEEDLINGS OF PHASEOLUS 69 
understandable the almost invariably twelve-bundled condition of the 
first epicotyledonary internode. 
The structure of the normal seedling thus corresponds to the type found 
by one of the writers^ 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 
leaf is trilacunar. 
The Trimerous Seedling 
The seedling with three cotyledons and three primordial leaves is built on 
a 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 seedHng. 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 vStrands 
bounded by a dotted line corresponds in origin and character to a similar group at this 
level in the dinlerous 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. 
^ Sinnott, E. W. Conservatism and variability in the seedling of dicotyledons. Amer. 
Jour. Bot. 5: 120-130. 1918. 
