CHAPTER 10 

 POLYEMBRYONY 



The phenomenon of polyembryony, 1 i.e., the occurrence of more 

 than one embryo in a seed, has attracted much attention ever since 

 its initial discovery in the orange by Leeuwenhoek (1719). Ernst 

 (1918) and Schnarf (1929), who have reviewed the older literature, 

 classify it into two types— "true" and "false" — depending on 

 whether the embryos arise in the same embryo sac or in different 

 embryo sacs in the ovule. This classification, although useful, is 

 open to some objections. It has been pointed out that while those 

 cases in which two or more embryos are formed as a consequence of 

 the development of aposporic embryo sacs are here classed under 

 the "false" type, others showing adventive embryos — which also 

 originate from tissues outside the embryo sac — are classed as 

 "true." To obviate this difficulty, Gustafsson (1946) has proposed 

 that the term false polyembryony should be restricted to those cases 

 only in which two or more nucelli, each with its own embryo sac, 

 fuse at an early stage. All others are included under true poly- 

 embryony. Since the first is only a teratological condition, we 

 may confine our attention to true polyembryony only. 



Cleavage Polyembryony. The simplest method of an increase in 

 the number of embryos is a cleavage of the zygote or proembryo 

 into two or more units. Although common in gymnosperms, its 

 occurrence is only sporadic in the angiosperms. Jeffrey (1895) 

 gave a detailed account of cleavage polyembryony in Erythronium 

 americanum (Fig. 194). After fertilization the synergids degenerate 

 and disappear, and the zygote divides to form a small group of cells, 

 which do not show any definite order or arrangement. This group 

 continues to increase in volume, and outgrowths arise at its lower 

 end which eventually function as independent embryos. The pro- 

 duction of two or three embryos from such an "embryogenic mass" 



1 For more detailed treatments of this topic see Webber (1940), Gustafsson 

 (1946), and Maheshwari (1950). 



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