THE TWO MAJOR TYPES OF ANGIOSPERM EMBRYOS 357 



"arising side by side" in the proembryo. The "terminal" position of the 

 cotyledon is attained by growth greater and more rapid than that of the 

 shoot apex. The plumule is definitely terminal and the cotyledon lateral 

 in some monocotyledons — several, probably all, genera of the Com- 

 melinaccae [CommcUnu, Tinautia (Fig. 125)], and the Dioscoreaceae 

 (Dioscorea, Tamtis), and in Trillium and Guzmannia. In the closely 

 related genera, Trillium and Paris, fairly primitive monocotyledons, 

 cotyledon and plumule position differ. In Trillium, the plumule is 

 terminal and the cotyledon lateral, as in most dicotyledons; in Paris, 

 the plumule seems to be lateral and the cotyledon terminal. In the 

 seedling in both genera, the strongly developed, persistent, green cotyle- 

 don is erect; in Trillium, it stands parallel with the plumule. On- 

 togenetically, the plumule arises terminally in both genera, but, in 

 Paris, it is pushed to one side bv the thickening of the hypocotyl. These 

 two genera show, in the relationships of hvpocotyl, cotyledon, and 

 plumule, a transition in cotyledon position from that of the dicotyledons 

 to that of the monocotyledons. Though in Paris, the cotyledon arises, 

 ontogenetically, before the stem apex, as in most monocotyledons, time 

 of origin cannot be considered evidence that the plumule is lateral, 

 because the cotyledon primordia of the dicotyledons also commonly 

 arise first. Morphologically, the cotyledon is lateral, as is the solitary 

 carpel of Acacia, Primus, Actaca, Bauhinia, and the solitary stamen of 

 Euphorbia, Myristica, Najas. (That these floral organs are truly lateral 

 is shown in various ways — by comparison with the same organs in re- 

 lated taxa, by ontogeny, by vascular anatomy.) 



Morphologically, the axis of a monocotyledonous embryo is not borne 

 laterally on the cotyledon, though, in the young embryo, it appears to 

 be initiated there and its development is later than that of the cotyledon. 

 The pseudoterminal position of the cotyledon is the result of the domi- 

 nance of a lateral cotyledon, which pushes the axis to one side by dif- 

 ferential growth, as shown in Paris; in many taxa, the so-called terminal 

 position is congenitally established. In those where a remnant of the 

 second cotyledon is unquestionably present, the terminal position of 

 the axis tip is clear. 



The monocotyledonous embryo has long been a morphological puzzle 

 and the subject of much controversy but, if the embryos of taxa now 

 generally accepted as primitive — some of the Liliales and Helobiales, 

 especially — are considered basic types, steps in the reduction and fusion 

 leading to the extreme forms found in the grasses are clear. 



Simplification is outstanding in the orchid embryo, where there is 

 little evidence of separate organs; the bulbous body of the embryo 

 seems to represent an enlarged axis, and the cotyledon only a minor 

 projection. Very small, undifferentiated embryos in endospermless seeds 



