ANATOMY OF THE EMBRYO AND YOUNG SEEDLING 349 



trace. Odd nvimbers clearly result from the fusion of two or more traces. 

 Number, position, and fusion of traces coincide with those of leaf traces. 

 A single trace is characteristic of highly specialized cotyledons; all 

 stages in lateral fusion are found — from fusion at the base only to 

 fusion throughout the length of the bundles. Different taxa show vari- 

 ous degrees of this fusion; commonly, the lateral bundles are free 

 distally and, with the median bundle, form a three-veined cotyledon. 

 The two free lateral bundles become major elements in the anatomy of 

 many specialized cotyledons. 



Characteristic of many monocot)dedons is the presence of two strong 

 lateral traces, which supply the sides of the sheathing leaf base. These 

 are prominent in cot)dedons in which the tip is bent downward. The 

 median vein, which supplies the cotyledon tip, bends downward to 

 the scutellum; the lateral veins may continue upward in the sheathing 

 wings or, rarely, also continue downward to the scutellum. The tip of 

 the median bundle, as it enters the scutellum, remains unbranched 

 in small, cylindrical, and club-shaped forms, but becomes variously 

 branched in large forms. In taxa with large amounts of endosperm, as 

 in the palms, an extensive vascular system may be built up as the scutel- 

 lum enlarges. 



In most cotyledons that have bA'O or three free traces, all the vascular 

 bundles continue as veins well toward the tip (Fig. 7). In the down- 

 bent cotyledons that have the tip transformed into a scutellum, all the 

 bundles may continue into the scutellum, but usually, only the median 

 extends to it; the lateral bundles are shorter, and their distal parts 

 recurve more or less below the point of down-bending. In down-bent 

 cotyledons with connate base and top, and where the sheath forms the 

 prominent part of the cotyledon, the lateral bundles are also prominent; 

 in those in which the median vein is reduced as a result of the conna- 

 tion, the lateral bundles may form the entire vascular supply of the 

 sheath and, therefore, of the coleoptile, its distal "elbow." 



cotyiedonary bundle with the vascular stele. E to H, Tigridia Pavonia, E, a younger 

 stage than F, both showing cotyiedonary bundle going up into the sheath and 

 back again (inverted) and on through the mesocotyl into the scutellum. Plumular 

 traces showing separation of cotyledon from hypocotyl. G, cross section of F at ij-tj 

 showing vascular stele and inverted bundle of adnate cotyiedonary neck; H, cross 

 section of F at x-x showing upward and downward (inverted) trace of coty- 

 iedonary bundle in the sheath. The prominent ridge in H showing evidence of 

 connation of sheath and neck and in G of adnation of hypocotyl and neck. I, J, 

 cross sections of mesocotyl, in Oryza sativa and Phalaris canadensis, respectively, 

 showing vascular stele of hypocotyl and inverted cotyiedonary bundle. K, Zea mays, 

 cross section of mesocotyl showing histological union of stele and cotyiedonary 

 bundle, cs, cotyiedonary sheath; h, hypocotyl; mes, mesocotyl; sc, scutellum. {A 

 to I, after Schlickitm; J, K, after Van Tieghem. ) 



