284 MINNESOTA BOTANICAL STUDIES. 



ling has two cotyledons, the primary root will be diarch ; if it 

 has three cotyledons, the root will be triarch, a condition which 

 Dangeard ('90) has noted in other gymnosperms. In the transi- 

 tion-region between root and stem, considerable diversity of 

 structure prevails among Ginkgo seedlings. Yet the numerous, 

 and seemingly very different types can be readily explained as 

 modifications of the one general plan illustrated conventionally 

 in the adjacent diagrams. 



Examining serial sections of a seedling of about the age of 

 those shown in figs. 42-46 we find, a short distance above the 

 root-meristem, two xylem-bundles appearing at the ends of the 

 elongated oval stele ; and in the sides of the latter, two long 

 arching bundles of protophloem (Diagram A and Jig. 50). In 

 each xylem-bundle the protoxylem lies away from, and the 

 metaxylem towards, the center of the stele (Diagram A). En- 

 tering the transition-region the protoxylem broadens and finally 

 divides into two masses which continue to separate as they pass 

 upward (Diagrams B and C). With the separation of the two 

 protoxylem groups the metaxylem is drawn out between them 

 into a broad plate (Diagram D), and in this condition the xylem- 

 mass passes out to the cotyledon (Jigs. 55, 56, 61). In the 

 petiole the metaxylem plate divides, each portion continuing to 

 rotate about its contiguous protoxylem until it reaches the posi- 

 tion shown in Diagram E. As the xylem-bundles pass on into 

 the blade of the cotyledon they continue to separate. The 

 metaxylem broadens out, appearing fan-shaped in section (Jig. 

 5(5?), and in the center of the cotyledon it nearly surrounds the 

 protoxylem, thus forming a mesarch bundle (Jig. jp) as de- 

 scribed by Worsdell ('97). With the first splitting of the pro- 

 toxylem-bundle, as it enters the transition-region, a proto- 

 phloem-group appears between and beyond its two points 

 (Diagram B). This protophloem-bundle increases in bulk as it 

 passes upward, and enters the cotyledon as a single bundle 

 (Diagram D and E). Later it divides to contribute a proto- 

 phloem-group to each bundle of the cotyledon (Jig. jp). 



In the lower portion of the transition-region, additional protox- 

 ylem-elements appear at various points between the cotyledon- 

 traces, and just inside the prospective cambium-zone. Farther 

 up, these scattered elements give place to usually six rather def- 

 inite strands, which continue upward as the protoxylem-groups 

 of the six primary bundles of the stem (Diagrams B to D and 



