i88 MORPHOLOGY OF GYMNOSPERMS 



cotyledon; that the bundles pass from mesarch to exarch while tra- 

 versing the cotyledons; that each cotyledonary bundle gives rise to 

 two protoxylem poles of the root; and that there may be an addition 

 of protoxylem elements after the root structure has been organized. 

 The cycadean type of seedhng is described (42) as consisting of 

 four bundles at the base of each cotyledon and a tetrarch root, which 

 generally becomes diarch below. Miss Thomas (48) has proved 



Fig. 213. — Ginkgo biloba: transverse section of cotyledon showing mesarch 

 character of vascular bundle; />, protoxylem; *, metaxylem; t, "transfusion tissue"; 

 Xiso. — After Spkecher (46). 



this cycadean type for the Ginkgo seedling by showing that the appar- 

 ently single bundle that enters the cotyledon is really a plexus of four 

 bundles, separating into two double bundles in the petiole, and each 

 of these separating into its two constituent bundles in the blade. It 

 is in the cotyledons that a very distinct mesarch structure is developed 

 (20). The relation of the number of protoxylem poles in the root 

 to the number of cotyledons (12, 39), that is, a diarch root related 

 to two cotyledons and a triarch root to three, is true only in an indirect 

 way. The original root is tetrarch, and is related to the four cotyle- 

 donary strands from the transition region, and the diarch condition 



