328 



MORPHOLOGY OF THE ANGIOSPERMS 



rank. But bilobed cotyledons occur in taxa scattered throughout the 

 angiosperms. Unequal bilobing of a remarkable type is present in the 

 highly specialized embryos of grasses and a few other families. 



The so-called terminal position of the cotyledon of the monocotyle- 

 dons (Fig. 122) has been used to support the std generis theory of 

 cotyledon nature, on the basis that a terminal cotyledon cannot be in- 

 terpreted as the morphological equivalent of a leaf, which is a lateral 

 appendage, and, lacking stem structure, it cannot be a part of the 

 stem. But both mono- and dicotyledonous taxa that have aberrant 



Fig. 125. Longitudinal sections of seeds Fig. 126. Longitudinal sections of de- 

 showing monocotyledon embryos with veloping embryos. A, a monocotyledon, 

 two cotyledons and terminal stem apex. Ottclia alismoides, and B, a monocoty- 

 A, Commelirui hirwinski/i; B, Tinantia ledonous dicotyledon, Claytonia vlrgin- 

 erecta. (After Sohns-Luubach.) ica, showing similarity when terminal 



position of plumule growing point p be- 

 comes pseudolateral by dominant de- 

 velopment of the pseudoterminal coty- 

 ledon c. {After Haccius.) 



embryo structure demonstrate that all cotyledons are, morphologically, 

 lateral organs. In the embryos of some monocotyledonous families — 

 Commelinaceae, Dioscoreaceae — the stem apex is definitely terminal in 

 position and in ontogeny (Fig. 125). It is apparent that the single 

 cotyledon, where dominant in the embryo, has crowded the axis and its 

 growing point to a lateral position (Fig. 126). 



The interpretation of the angiosperm cotyledon as basically "peltate" 

 is an extension of the similar interpretation of floral organs as peltate. 

 In dicotyledons, the cotyledonary petiole, like the leaf petiole, is fre- 

 quently grooved, sometimes even tubular, and simulates ascidiform 

 structure. A pair of cotyledons, united by their petioles, may also sug- 

 gest "peltate" form, but the only cotyledons that show evidence of true 

 peltate form are those of taxa where the leaves are strongly peltate, as 



