ANATOMY OF THE EMBRYO AND YOUNG SEEDLING 333 



with the vascular tissue of the hypocotyl, its free tip, which suppHes the 

 scutelhim, is attached at the base of the hypocotyl. This position is 

 obviously secondary, as is shown by a series of taxa with adnate cotyle- 

 don neck (Fig. 129F, G, H). In a commonly held interpretation of the 

 monocotyledon embryo, using Zea as an example, the scutellum repre- 

 sents the entire cotyledon; the mesocotyl is the first internode of the 

 stem; the coleoptile, the first leaf; and the hypocotyl, a mere plate of 

 tissue. But critical comparative study of monocotyledon embryos shows 

 that the cotyledon is divided transversely by adnation into thi'ce parts; 

 the free, distal part, the coleoptile; the free, basal part, the scutellum; 

 and the median section, which forms the lateral part of the mesocotyl, 

 is, superficially, lost by adnation. Failure to understand this complex 

 structure has been responsible for the interpretation of the coleoptile 

 as the first leaf and the hypocotyl as the first internode. The incorrect 

 interpretation of homology is not surprising, because the morphological 

 base of the cotyledon is distal on the embryo and the apex is borne 

 proximally. So complete a "breaking apart" and apparent isolation of 

 the parts of an organ probably occurs nowhere else in the angiosperms. 

 It represents an extreme modification resulting from the adoption of 

 hypogeal germination. Here, as in flower specialization, the monocotyle- 

 dons seem to have surpassed the dicotyledons in specialization. The use, 

 in teaching, of the embryo of Zea as a typical monocotyledon embryo 

 is most unfortunate; this embryo is one of tlie most highly specialized 

 in angiosperms. 



The complex nature of the mesocotyl, internally obvious where the 

 cotyledon bundle lies free in the "cortex" (Fig. 134B, G, I, J), is ob- 

 scure or hidden where the bundle is united with the stelar vascular 

 strands. The presence of the inverted bundle in the "cortex" of the 

 hypocotyl was noted and illustrated several times in the later decades 

 of the nineteenth and the first decades of the t\ventieth century, but, 

 though well illustrated, it has been overlooked recently. If its presence 

 had been evaluated, it could hardly have been urged that the term 

 mesocotyl is superfluous. 



Anatomy of the Cotyledonary (Stipular, Ligular) Sheath. The term 

 cotyledonary sheath has been loosely applied both to the caplike or 

 sheathlike structure, the coleoptile, which encloses the plumule, and 

 to the basal part of the cotyledon. That the cotyledonary sheath is, as 

 the term suggests, a part of the cotyledon, has been generally recog- 

 nized, but it has rarely been made clear what part of the cotyledon it 

 represents. Anatomy supports the interpretation that the sheath is the 

 basal part of the cotyledon; the neck, the median part, with the angular 

 "elbow" region differentiated as the coleoptile; and the apex, the 



