342 MORPHOLOGY OF THE ANGIOSPERMS 



Difficulties of interpretation of the monocotyledonous embryo are in- 

 deed great, and confusion in description is not surprising. The extra- 

 ordinary form of a cotyledon that is divided into two parts isolated in 

 their attachment on the axis, with the distal part of the cotyledon at- 

 tached below the proximal part, is, in itself, remarkable. And with this 

 separation in position goes difference in form and function. 



Failure to recognize the presence of adnation — regardless of the clear 

 anatomical evidence (Fig. 134) — is largely responsible for the delay in 

 recognition of the basic structure of these embryos. Yet textbooks of the 

 nineteenth century, especially those published from 1860 to 1880, de- 

 scribed the cotyledon of the monocotyledons as consisting of the coleop- 

 tile and the scutellum. This division of the cotyledon was demonstrated 

 by comparative study of form and by anatomy in 1872, 1881, 1885, 1887, 

 1896, 1899, 1915, but has been otherwise interpreted since 1915. The 

 earlier interpretation is the correct one. 



Relation of the Cotyledon of the Monocotyledons to the Embryo Axis. 

 Although the cotyledons are recognized as lateral appendages of the 

 axis in the dicotyledons, the cotyledon of the monocotyledons is com- 

 monly called terminal. If the soHtary cotyledon is, morphologically, the 

 equivalent of one of the two in dicotyledons, its apparently terminal 

 position must be secondary, and comparison of embryos throughout the 

 monocotyledons shows this to be so. In some families — the Dioscoreaceae 

 and Commelinaceae — and in scattered genera elsewhere, the stem apex 

 is clearly terminal and the cotyledon lateral (Fig. 125). In most mono- 

 cotyledons, the surviving cotyledon has become dominant in the embryo 

 and crowds the axis to one side. In some taxa, the lateral position of 

 the axis is assumed ontogenetically, but the pseudoterminal position of 

 the cotyledon has become established phylogenetically in most taxa. 

 The "'terminal" position has been used as evidence that the cotyledon 

 is not the morphological equivalent of the leaf, but other organs — 

 carpels, stamens, and ovules — similarly appear to be terminal when not 

 so. It is argued that, in differentiation from the proembryo, the cotyledon 

 develops first and terminally, and the axis appears as a later, lateral 

 structure. But in some genera — Sparganiiim and Pistia — the first plum- 

 ular leaves also appear before the stem apex. This has led to morpho- 

 logically remarkable interpretation that, in these genera, the first leaves 

 are cauline, the later ones, lateral appendages — a morphological 

 absurdity. 



Relation of the Solitary Cotyledon to the Pair of Cotyledons. One the- 

 ory of the relationship of the one- and the two-cotyledon embryos is 

 that the single cotyledon represents the primitive condition and the 

 two cotyledons have arisen by a splitting of the ancestral solitary one. 

 The presence of two traces and of two, often opposed, vascular bundles 

 in the lamina of the single cotyledon of Anemorrhea and other genera 



