390 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



same species at different stages of their development. The pH 

 value of the culture medium is an additional factor that has to be 

 kept in mind, a pH of about 6.0 having been found most favorable 

 for Datura. Regarding the constituents of the medium, Sanders 

 reports that in her experiments Seitz -filtered malt extract gave a 

 definite increase in growth values, while the addition of diastase or 

 auxins was of no advantage. 



In conclusion it may be said that the artificial culture of embryos 

 is important in several ways. It is the method par excellence for 

 understanding the nutritive requirements of the developing em- 

 bryo ; it gives us an insight into the factors that influence embryonic 

 differentiation; and it promises to be of great economic value as a 

 means of achieving a much wider range of hybrid combinations 

 than has been possible up to this time. Further, seeds which 

 normally remain dormant for several weeks or months can now be 

 made to germinate not only without giving the embryos a period 

 of rest but even before they have become mature. 6 To give a 

 single example, Iris seeds, which normally germinate only after two 

 to several years, can now be made to yield seedlings in the same year, 

 so that the breeder saves himself the uncertainty and delay which 

 troubled him previously (Randolph, 1945). 



There are two chief limitations in exploiting the embryo culture 

 method to the fullest advantage. First, young embryos, especially 

 of hybrids, often fail to grow in artificial media. Second, excised 

 embryos tend to germinate immediately to produce miniature seed- 

 lings rather than to continue their usual growth and attain full 

 differentiation. This premature germination results in curious 

 growth patterns and weaker seedlings. 7 For a solution of these 

 difficulties we require a better knowledge of the nutritive require- 

 ments of the embryos in terms of known chemical substances. Fu- 

 ture research will no doubt help to clear some of the present obscuri- 

 ties in this connection and may well enable us not only to obtain an 

 uninterrupted growth of embryos in artificial media but also to fol- 

 low the entire process of fertilization and embryogeny in a petri dish. 



6 Since excised embryos are able to skip this "after-ripening" or "resting" period 

 in culture, this method has recently been used to make quick tests of the germinabil- 

 ity of peach seeds used by nurserymen (Tukey, 1944). 



7 See Kent and Brink (1947) for some suggestions for bringing about a continua- 

 tion of the embryonic type of growth. 



