352 MORPHOLOGY OF THE ANGIOSPERMS 



region of the dicotyledons, like that of most conifers, has been viewed 

 as derived from the complicated type of the cycads and the araucarian 

 conifers. No conclusions can be drawn, however, as to the primitive 

 type, because of the comparatively small amount of information about 

 the transition region in angiosperms. A correlation of number of pro- 

 toxylem poles in the hypocotyl with the number of cotyledon traces, in 

 the light of fusion and reduction of traces, should be a help in deter- 

 mining the basic type or types in angiosperms. 



Anatomy of the Mesocotyl. The mesocotyl is a compound structure 

 made up of the hypocotyl and the adnate part of the cotyledon (Figs. 

 130 and 134A, F); it contains vascular tissues of both organs — the stele 

 of the hypocotyl and one or more vascular bundles of the cotyledon 

 (Fig. 134B, G). Stages showing the external and internal effect of adna- 

 tion of the cotyledon neck to the hypocotyl are frequent. External 

 evidence of the fusion is the presence, in some genera, of a longitudinal 

 ridge on the embryonic axis (Fig. 134G); internal evidence is the 

 presence of a vascular bundle running longitudinally in the cortex of 

 the hypocotyl. The "superfluous" bundle belongs to the downbent adnate 

 neck of the cotyledon ( Fig. 134A, E, F ) ; it is the vascular supply of tlie 

 scutellum, the median bundle of the cotyledon continued downward 

 from the point of downbending of the cotyledon neck. The vascular 

 supply of the scutellum thus follows a "roundabout" course, like an in- 

 verted V (Fig. 134F, F). As a result of the bending downward of the 

 cotyledon tip, the adnate vascular bundles, as seen in cross sections of 

 the mesocotyl, are inverted, the xylem external to the phloem (Fig. 

 134B, G, H). (The presence of an inverted bundle in the cortex of the 

 hypocotyl, apparently an anatomical anomaly, is a structural feature 

 resulting from the adoption of hypogeal germination.) 



Where adnation of the cotyledon and the hypocotyl is histologically 

 more intimate, the cotyledonary bundle lies still deeper in the pericycle 

 (Fig. 1347, 7), or in contact witli the vascular cylinder of the hypocotyl 

 (Fig. 134D). In the most highly specialized embryos, it may even be 

 fused with one of the bundles of the hypocotylary stele, forming a con- 

 centric bundle, or be imited laterally with the vascular cylinder, losing 

 its identity (Fig. 134K). (Histological union of vascular tissues of 

 different organs lying side by side represents the highest degree of 

 anatomical fusion; it is seen frequently in other parts of the plant — 

 inflorescence to stem, flower to leaf, especially of floral organs to one 

 another. But fusion of vascular bundles of different organs is nearly 

 always between similarly oriented vascular bundles; in the mesocotyl, 

 the vascular bundles are differently oriented.) 



Where the "downrunning" cotyledonary bundle is histologically merged 



