Feb., I92ll HARRIS AND OTHERS SEEDLINGS OF PHASEOLUS 
67 
with the protoxylem in an endarch position, begins to be assumed. Each 
pair is subsequently referred to as a "primary double bundle." Thus the 
level of transition from root structure to stem structure is low, being prac- 
FiG. I. Dimerous seedling. Transverse section through the root, showing its 
tetrarch condition (four protoxylem poles). Fig. 2. Dimerous seedling. Transverse 
section through the base of the hypocotyl showing the four primary double bundles, each 
of which has been derived from one of the four root strands. Fig. 3. Dimerous seedling. 
Transverse section through the mid-region of the hypocotyl showing the normal eight- 
bundled condition. No intercalary bundles are figured. Fig. 4. Dimerous seedling. 
Transverse section just below the cotyledonary node. The four bundles or bundle groups 
have originated by a more or less complete fusion of the adjacent members of each of the 
original pairs. Each bundle, as shown by the two constrictions in it, is about to break up 
into the three strands shown in figure 5. Fig. 5. Dimerous seedling. Transverse section 
through the cotyledonary node. Each group of three strands which have arisen by a 
breaking up of the large bundles in figure 4 is here enclosed by a dotted line. These three 
strands are a cotyledonary trace (solid black), an epicotyledonary bundle, and a small 
bundle which will fuse with its adjacent neighbor to form another epicotyledonary bundle. 
Fig. 6. Dimerous seedling. Transverse section through the mid-region of the epicotyl 
showing the twelve bundles which have arisen by the splitting of the six original epicoty- 
ledonary bundles. The six strands which are to go off as traces to the two primordial 
leaves are solid black. 
tically at the base of the hypocotyl. The members of each of these four 
pairs soon separate until the eight bundles are approximately equidistant 
(fig. 3), a condition which persists throughout the hypocotyl until the coty- 
ledonary node is approached. 
In addition to these bundles, there are in a considerable percentage of 
the normal seedlings studied a variable number of accessory or intercalary 
bundles, the " Zwischenstrange " of Dodel. These may make their appear- 
