346 MORPHOLOGY OF THE ANGIOSPERMS 



phyte — nucellus or integument — or from the zygote, proembryo, or sus- 

 pensor; the gametophyte types arise from cells of the embryo sac other 

 than the fertilized egg. The term polyembryony — true polyembryony — 

 is probably best applied only to the presence of more than one embryo 

 derived from one fertilized egg. Embryos that arise by budding from 

 tissues of the mother sporophyte are called advcntive. (All supernu- 

 merary embryos are often called adventive. ) In origin, true adventive 

 embryos resemble adventive leaves and other organs. Embryos may 

 arise by splitting from the body of the embiyo; one or two develop 

 laterally on the suspensor when it is massive. The origin of more than 

 one proembryo directly from the zygote is rare. 



Gametophytic embryos arise from synergids or antipodals; those 

 from synergids may be of doubtful nature, for synergids, as sister cells 

 of the egg, may have been fertilized. The place of origin of embryos 

 developing within the embryo sac is often difficult to determine, be- 

 cause embryos that arise outside the sac protrude into the sac as they 

 develop. 



Polyembryony is common in some of the gymnosperms and has been 

 found in many angiosperms in families scattered throughout the taxon. 

 It may be a common character in a genus, as in Citrus, where, in some 

 species, it is present in nearly every ovule, or it may occur in only an 

 occasional ovule. Growth conditions perhaps have some influence on 

 its development. In many families, no polyembryony has been reported. 



Adventive embryos of nucellar origin are more common than those of 

 integumentary origin; they are found in many dicotyledons and in a 

 few monocotyledon families — Liliaceae, Amaryllidaceae, Orchidaceae, 

 Araceae, Gramineae. Nucellar polyembryony has been described as 

 present in about 19 per cent of the ovules of Trillium iinduJatum but 

 the additional embryos usually degenerate. Integumentary embryos — 

 derived from the integument only — are found mostly in the dicotyledons; 

 in the monocotyledons, they are known only in the Liliaceae and Amaryl- 

 lidaceae. Polyembryony is perhaps best known in Citrus, Mangifera, 

 Opuntia, Nicotiuna, Fiinkia, Lilium, Erijthroniiim, Allium, Poa. 



Occasionally, more than one embryo may be formed in an ovule in 

 which two or more embryo sacs exist, either from two megaspores 

 from the same mother cell, or from multiple megaspores developed 

 from multicellular archesporium. Rarely, two ovules may be fused, 

 simulating one. The term pseudopolyembryony has been applied to the 

 development of multiple normal embryos from fertilized eggs of more 

 than one embryo sac in an ovule, reported as occasional in Trifoliiim, 

 Rosa, Saxifraga. The "polyembryony" of Prunus, Qucrcus, Castanea, and 

 other drupes and nuts is based on the error of interpreting seeds as 

 embryos. It has been suggested that polyembryony may be a primitive 



