360 MORPHOLOGY OF THE ANCIOSPERMS 



tion made that the family should be transferred to monocotyledons. But 

 it was shown that Nijmphaea has two cotyledons and that Niiphar, 

 which has one of its two cotyledons reduced in size, shows stages in 

 fusion of the cotyledons — basally in some species, by one margin in 

 others. In Nehimbo, the single cotyledon consists of two, fused to form 

 a cup-shaped, two-lobed structure surrounding the plumule. The lobing 

 was interpreted as showing derivation of two cotyledons from one by 

 division, but anatomy shows that the lobing is the result of basal fusion 

 of the two cotyledons, a condition frequent in dicotyledons — PodophyJ- 

 lum, Eranthis, Ranunculus. 



In the dicotyledonous genus, Peperomia, the geophilous species show 

 stages in the origin of monocotyledony (Fig. 136). Most species of 

 Peperomia have two cotyledons, strictly opposite, which, at germina- 

 tion, are withdrawn from the seed and become erect, photosynthetic 

 organs. But the minute, cormous species show stages in the modification 

 of one of the pair to form a suctorial organ. These stages are shown 

 diagrammatically in Fig. 136. P. peUucida has two normal green cotyle- 

 dons; P. peruviana has one photosynthetic cotyledon and the other, 

 though well-developed, remains within the seed; in P. parvifolia, the 

 cotyledon remaining within the seed has become club-shaped and 

 forms a typical absorbent organ. Noteworthy in the early stage of re- 

 tention of one cotyledon is the unreduced size of this cotyledon and 

 the large cavity where the photosynthetic cotyledon lay (Fig. 136D). 

 Where the retained cotyledon has become a specialized suctorial organ, 

 the photosynthetic one is small, and the absorbing organ fills a small 

 cavity in the endosperm. The elaboration of the suctorial organ is com- 

 pleted in P. parvifolia. The intermediate stage is similar to that in some 

 araceous genera. Parallel stages in the development of the retained, 

 suctorial cotyledon exist in dicotyledons and monocotyledons. (The 

 suctorial organ of most monocotyledons has an origin different from 

 that of the dicotyledons.) 



Aberrant Embryos in the Monocotyledons. Aberrant embryos seem to 

 be less frecjuent in the monocotyledons than in the dicotyledons. From 

 these monocotyledons comes some of the best evidence of derivation 

 from dicotyledonous ancestors. In genera of the Dioscoreaceae — 

 Dioscorea, Trichoptis, Rajania, Tamus — and in the Commelinaceae 

 [Commelina, Tinanfia (Fig. 125)], the second cotyledon is present in 

 reduced form. Sagitfaria has been described as sometimes showing evi- 

 dence of a lost, second cotyledon, a projection with a vascular trace on 

 the hypocotyl opposite the normal cotyledon. Some taxa in the Araceae 

 — Arum, Arisacma — and some Liliaceae — Trillium, Paris — have both 

 cotyledons well developed, standing opposite each other at the top of 

 the hypocotyl, one foliaceous and commonly interpreted as the first 



