394 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



ture, many attempts have been made to produce it experimentally 

 but without any marked success up to this time. 



Haberlandt (1921, 1922) made the observation that in natural 

 adventive embryony the proliferation of the embryo-initiating cells 

 is invariably preceded by a degeneration of some of the adjoining 

 cells. He was led by this to put forward the so-called "necrohor- 

 mone theory," according to which the stimulus for cell division 

 and proliferation is supplied by certain substances liberated from 

 the adjacent degenerating cells. Proceeding on this basis, he tried 

 to produce adventive embryos in Oenothera by pricking the ovules 

 with a fine needle or by gently squeezing the ovary so as to damage 

 the cells slightly. In one ovule he obtained two embryos, which 

 he considers to be of nucellar origin (Fig. 210). 



In repeating Haberlandt 's technique, Hedemann (1931) obtained 

 a two-celled embryo and a free nuclear endosperm in an unpol- 

 linated ovary of Mirabilis uniflora which had been pricked with a 

 fine insect needle. No chromosome counts could be made, how- 

 ever, to ascertain whether the embryo was haploid or diploid, and 

 the mode of its origin (whether from the egg or the nucellus) does 

 not seem to have been conclusively established. 



After Hedemann, no other worker has reported any success in 

 the artificial production of adventive embryos by Haberlandt 's 

 methods, and Beth (1938), who made several unsuccessful attempts 

 with Oenothera and other plants, denies the nucellar origin of the 

 embryos even in Haberlandt 's material. 11 He considers that in 

 Haberlandt 's experiments emasculation either was incomplete or 

 had been performed too late, and that the embryos arose from an 

 accidental fertilization of twin embryo sacs. 12 



Recently, Van Overbeek, Conklin, and Blakeslee (1941) injected 

 several chemical substances into the ovary of Datura stramonium in 

 the hope of inducing parthenogenetic development of the egg cell. 

 This attempt was unsuccessful, but they obtained instead, on in- 

 jection of a 0.1 per cent solution or emulsion of the ammonium 

 salt of naphthaleneacetic acid or indolebutyric acid, several multi- 

 cellular warty outgrowths which filled the embryo sacs (Fig. 211). 



11 See also criticism by Gustafsson (1947) who concludes that there is no evi- 

 dence, experimental or morphological, to show that adventive embryony is induced 

 by substances from dying cells. 



12 As mentioned on pp.96,97 twin embryo sacs frequently occur in the Onagraceae. 



