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immediate germination, as shown by experiments with excised embryos 

 in vitro and by germination tests on unripe seeds. The interest in the 

 tissues surrounding the embryo now centers on the question of why an 

 embryo able to germinate does not do so, and why instead it continues 

 to grow embryonically until it is fully mature. This question is considered 

 in a recent review paper by Evanari (5). A number of factors have been 

 found which may prevent premature germination. These include high 

 osmotic pressure, unfavorable pW, and a variety of chemical substances 

 such as ethylene, organic acids, unsaturated lactones, aldehydes, essential 

 oils, and alkaloids. They occur in all parts of plants, including the seed 

 coat, endosperm, and the embryo itself. Their effects are mostly non- 

 specific, although there are variations in the strength of the inhibition 

 response and in the subsequent reaction of the posttreatment scedhng. 

 The second approach to the whole problem of plant embryo growth is 

 that of experimentation with artificial media, that is, the cultivation 

 of plant embryos in vitro. It is generally true that any plant embryo, if 

 old enough at the time of excision, may be successfully cultivated on a 

 medium containing only water, agar, minerals, and sugar. Qjnversely, 

 it has been found that the embryos of all species which have been tried, 

 if young enough when excised, will fail to grow on any artificial medium 

 now known. The experiments to be discussed here concern embryos 

 between these two age limits. Van Overbeek, Blakeslee, and Conklin 

 (11,12) found that growth of immature Datura embryos was benefited 

 by the addition to the medium of an arbitrary mixture of growth factors 

 including glycine, a number of vitamins, and several other organic 

 nutrients. These supplementary substances, however, did not aid in 

 the growth of embryos which were less than about .5 mm. long when 

 excised. These still failed to develop. The further addition to the medium 

 of unautoclaved coconut milk, a convenient source of endosperm, made 

 it possible to grow undifferentiated embryos as small as .1 mm. long. 

 Further evidence led to the conclusion that there are two factors present 

 in coconut milk: a heat stable factor (probably auxin) causing growth 

 but not differentiation, and a heat labile factor which induces differentia- 

 tion. The material responsible for the latter effect has not been identified, 

 but Blakeslee and Satina (i) have since found that a factor of comparable 

 activity is present in malt extract. Haagen-Smit and co-workers (6), 

 testing these findings on other material, found that coconut milk was 

 not a limiting factor for the growth of excised maize embryos. Embryos 



