354 MORPHOLOGY OF THE ANGIOSPERMS 



scutellum. The cotyledons of taxa that have mesocotyls are, in part, 

 "bottom-side-up" in relation to their apparent positions on the axes of 

 the embryos. 



In cotyledons in which there is little or no downbending of the distal 

 part, all the vascular bundles, including those of the sides of the sheath, 

 usually continue into the scutellum. These cotyledons are usually of 

 simple, primitive type, with two or four vascular bundles. In cotyledons 

 of advanced type, with one trace which divides to form a median and 

 two strong lateral bundles, only the median bundle continues to the 

 scutellum. The branches extend first upward and laterally and then 

 commonly downward, on the opposite side of the sheath. The pair of 

 arching, recurving bundles forms a prominent feature of the sheath in 

 many taxa, and their continuation over the top of the coleoptile has 

 entered into theories of the nature of the coleoptile. The course of the 

 bundles has been considered remarkable and evidence that the coleoptile 

 is an independent organ, not a part of the cotyledon. (The venation 

 type of the specialized cotyledon — one trace, representing three fused 

 laterally at the base, and forking to form a median and two lateral 

 branches — is similar to that found in advanced types of carpels, petals, 

 and leaves. ) The position and course of the veins of the sheath support 

 the interpretation that at least the upper part of the sheath is stipular 

 (ligular). In the evolutionary development of the coleoptile, fusion of 

 the "shoulders" — rounded stipule tips or auricles — continued distally the 

 closed sheath of the primitive cotyledon, forming the cap-shaped pro- 

 tective "tip" of the coleoptile (Fig. 130). The arching veins show the 

 make-up of the enclosing coleoptile. (Venation in auriculate stipules is 

 frequently arching.) 



Where there is adnation of the neck of the cotyledon laterally to its 

 lower part, the midvein is bent back upon itself. The two parts are fused, 

 and a bicollateral bundle (xylem both internal and external to the 

 phloem) is formed. Stages in this fusion are frequent. Under continued 

 specialization of this vascular fusion, where conduction "goes up and 

 back" in a double bundle, the bundle is shortened distally and may 

 become united with the vascular stele of the hypocotyl (Fig. 134D, K). 

 The shortening and adnation may extend to the point of origin of the 

 branches of the trace of the sheath, and the branches then appear to 

 arise directly from the hypocotyl. The origin of the trace of the sheath, 

 apparently directly from the hypocotyl, has been part of the evidence 

 that the sheath and the coleoptile represent the first leaf. The under- 

 standing of the downbending and fusion of the distal part (neck) of 

 the cotyledon to its basal part explains the "basal" (morphologically 

 false) position of the scutellum and the inversion of the distal part of 

 the cotyledon. 



