THE EMBRYO 331 



which it forms a ridge on the embryonic axis ( Fig. 134G, H ) . The com- 

 pound structure, so formed, is the mesocotijl (Fig. 134A, B). The ex- 

 ternally complete separation of the two parts of the cotyledon, base 

 and tip, is an extraordinary modification of an organ, the more so be- 

 cause the tip of the cotyledon, the scutellum, is lateral at or near the 

 base of the axis, and the sheathing base of the cotyledon, the coleoptile, 

 above it, covers the apex of the axis (Fig. 130). Only some of the 

 highest grasses and sedges and some of the Bromeliaceae are known to 

 have this extreme modification of the cotyledon. (With the great re- 

 duction of the flower and inflorescence and the anemophilous pollina- 

 tion of the grasses, this modification is substantiating evidence of the 

 high specialization of the Gramineae. ) 



The term scutellum is commonly used to designate the highly 

 specialized part of the cotyledon of many monocotyledons, the distal 

 part, which is structurally and functionally modified as a sti-ucture that 

 absorbs food materials stored in the seed outside the embryo (Figs. 129 

 C-H and 130). All stages in its evolutionarv development are found, but 

 the term is usually restricted to its most highly specialized form, an 

 absorbing part, which, especially in the grasses, is more or less shield- 

 shaped. There are many ti'ansitional forms between the earliest stages 

 of development of an absorbing part and its highly elaborated form. 

 Cotyledons, as well-defined absorbing organs, are present in many 

 monocotyledonous families, but in only few dicotyledons. The term 

 scutellum is sometimes defined as the absorbing cotyledon of the 

 grasses. This is an inaccurate definition, because the scutellum is only 

 a part of the cotyledon and is not morphologically distinct from the 

 absorbing structures of other families from which it differs only in 

 shape and size. If the term is to be restricted on the basis of form and 

 prominence in the embryo, it should be applied also to similar struc- 

 tures in some of the sedges and, perhaps, of other families. 



The Liliaceae show early stages in the transformation of the cotyle- 

 don tip into a suctorial structure. In Paris, the cotyledon apparently 

 has no suctorial function; the entire cotyledon becomes photosynthetic 

 (Fig. 128E). In the related Trillium (Fig. 128G), the tip of the cotyle- 

 don remains within the seed at germination, functioning as a suctorial 

 structure; the rest of the cotyledon is withdrawn from the seed and 

 becomes leaflike. As the cotyledonary petiole elongates and becomes 

 erect, the suctorial tip is withdrawn but withers without becoming 

 green. In Erythronium (Fig. 128F), the mesophyll cells of the cotyledon 

 tip differ in form and arrangement from those lower down and lack 

 chlorophyll, but the tip does not wither after withdrawal from the seed. 

 Similar early stages in modification of the cotyledon tip are present in 

 other primitive monocotyledonous families. In Triglochin, the tip is 



