THE ANGIOSPERMAE 



1479 



AJchemilla and Potentillo, where there is a multicellular archesporium, 

 there may also be multiple embn,'0 sacs which are not aposporic. A number 

 of archesporial cells enlarge and become mother cells and may develop 

 into embryo sacs either with or without meiosis. In spite of this there is 

 seldom polyembryony. 



The genus Citrus has long been known for the frequency of polyembry- 

 ony in the ripe seeds. This is associated usually with the formation of 

 adventitious embryos from nucellar cells around the embryo sac (Fig. 

 1355). Two or three embryos are commonly formed, but more than ten 

 may occur in one seed, some of them very small. Competition eliminates 

 some at earlv stages. Pollination seems to be necessary, even though, in 



Fig. 1355. — Citrus amantium. Polyembryony. A, Several embryos of different sizes in one 

 seed. {After Penzig.) E, Embrvo sac with normal embr\o (right) and a group of 

 adventitious nucellar embryos (left). (After Strasburger.) B to D, Esenbeckia grandi flora. 

 Three embryos from one seed. {After de Jitssieii.) 



some varieties, the pollen may be "foreign". A normally fertilized embryo 

 is generally present and the fertilized oosphere may occasionally split to 

 yield true twins. It is notable that the adventitious embryos are often 

 clustered round the sexual embryo as if it provided a stimulus to their 

 development. The formation of endosperm, following fertilization, seems 

 to be essential if any of the embryos are to complete their development. 

 Whether adventitious embryony is to be regarded simply as one method 

 of vegetative propagation or not, raises difficult questions. In Citrus, at 

 any rate, long continued propagation by cuttings leads to senility, while 

 reproduction from adventitious embryos does not. The following genera 

 may be added to Citrus as showing adventitious embryony conspicuously: 

 Mangifera, Opuntia, Eiionymus, Finis, Chisia. 



The causes of sexual sterility in Angiosperms are so varied that no 

 simple theory of the origin of apomixis can be given. That it is closely 

 related to polyploidy is well established. So is the perennial habit, to which 

 vegetative means of propagation are related. Direct observation tends to 

 confirm the conclusion that vegetative propagation and seed fertility are 

 inversely related. 



