308 



EMBRYOGENESIS IN PLANTS 



and plasmatic growth — becomes separated by an intercalated section 

 of incipient cell elongation.' The period before this elongation of tissue 

 sets in includes the pregerminal phases. 



The Stages of Development of the Embryo in Ovulo 

 (after Rijven) 



When freshly excised, young embryos are green from the heart-shaped 

 stage onwards and contain some starch, though starch is not present 

 before this stage. Mature embryos have an ivory-white appearance; 

 their cells contain no starch but abundant aleurone grains and fats. 

 As from the 'torpedo' stage {see Table), the starch grains are large and 

 numerous and are mainly located in the hypocotyl; fats also begin to 

 be present from about this stage (Fig. 83). From the globular to the 

 fully-grown embryo, there is almost a hundred-fold increase in length. 

 Embryo length plotted against time through the embryonic develop- 

 ment yields an almost perfect sigmoid growth curve. 



Ovules containing immature embryos are highly turgid, the endo- 

 spermic sap, which contains naked nuclei and their enveloping cyto- 

 plasm, exuding freely on puncturing. Sap pH was found to be 6-0, its 

 osmotic value at the torpedo stage being isotonic with § mol. mannitol 

 (8-4 atm.). Sugars and starch are present in the sap and aminoacids 

 were also detected. The cells of the embryo may be assumed to be in 

 equilibrium with the embryo sac sap: hence embryos placed in distilled 

 water show a marked initial swelling. 



Globular embryos, i.e. less than 100//, have not been successfully 

 grown in culture, the heart-shaped stage (stages III and IV) being 

 critical in this respect. This is an important stage in the morphogenetic 



