THE EMBRYO 341 



critical comparison of vascular structure demonstrates the fusion — Zefl, 

 Sorghtim, Coix. The body of the embryo, though apparently simple, is, 

 morphologically, more than an axis; in the genera that have meso- 

 cotyls, it is partly cotyledon. 



The coleoptile was probablv first considered cotyledonary in 1849, 

 when it was noted that it differed from the plumular leaves in having 

 two vascular bundles, rather than the odd number characteristic of the 

 leaves. This interpretation was general in the middle of the nineteenth 

 century, when prominent botanists recognized that scutellum and 

 coleoptile constituted the cotyledon. The scutellum was called the 

 "terminal part" of the cotyledon, "retained in the seed as an organ of 

 absorption," even though it was noted that, in some of the grasses, this 

 organ is borne "above the first internode." The early recognition of the 

 basic morphology of the cotyledon is an example of the keen insight in 

 morphological interpretation of the botanists of the middle and later 

 nineteenth century. 



The Mesocotyl. The hypocotyl, with the downbent, adnate top of the 

 cotyledon, forms the mesocotijl (Fig. 130). (See also description under 

 coleoptile.) The axis of the embryo of some highly specialized grasses, 

 sedges, the bromeliads is, except at its tips, a mesocotvl. It is espe- 

 cially long and well developed in Leerzia, Orijza, Coix, Paiiiciim, Corex, 

 Zizanio, Cijpcrtis. It has been suggested several times that the term 

 mesocotyl be dropped as superfluous and meaningless. Under the inter- 

 pretation of the coleoptile as the first leaf, and of the axis below it as 

 the first internode of the stem — one interpretation of some grass embryos 

 — the term would be superfluous, but the coleoptile is surely not the 

 first leaf, and this structure is not a simple axis. A term is necessary for 

 this compound structure, and, though "mesocotyl" may be morphologi- 

 cally inappropriate, because it is more than the middle part of the 

 cotyledon, it has long been in use. The complex nature of the mesocotyl 

 was recognized as early as 1872, when, on anatomical evidence, a part 

 of the axis was shown to be neither^ root, stem, nor hypocotyl and was 

 described as an "elongated node." In the 1890s, it was called by some 

 authors the "Zwischenstiick," a part of the axis with an accessory vascular 

 supply. Its true nature — the result of adnation of part of the cotyledon 

 to the hypocotyl — was demonstrated by anatomical structure in 1915. 

 More recent interpretations have not given critical attention to com- 

 parative anatomy and the results of fusion of adjacent vascular bundles; 

 the critically important vascular structure of the mesocotyl has been 

 overlooked or dismissed as meaningless. 



A mesocotyl is characteristic of several genera of grasses, a few sedges 

 — Corex, Cypenis — and some of the Bromeliaceae — Aechmea, Billhergia. 

 In all these families, and perhaps in others, more examples will doubt- 

 less be found. 



