EMBRYOGENESIS IN FLOWERING PLANTS 315 



contents of their incompatible hybrids did not show the presence 

 of nucleic acids, although they strongly inhibited embryo growth. 

 Although nucleic acids may have an inhibitory effect on embryo 

 growth similar to that of ovular tumour extract, it is considered that 

 they are probably not truly alike in their action. 



In a comparison of the effects of nucleic acids and extracts of 

 ovular tumours on the development of Datura embryos, Rappaport 

 (1950) found that both ribonucleic and desoxyribonucleic acid, when 

 injected into young selfed D. stramonium capsules, produced an 

 inhibitory effect apparently similar to that caused by injections of 

 extracts of ovular tumours. The endosperm disintegrated and the 

 embryonic development was strongly retarded or completely inhibited. 

 Higher concentrations of these substances usually caused the death of 

 the ovules, ribonucleic acid showing greater effects at lower concen- 

 trations than desoxyribonucleic acid. When ribonucleic acid was added 

 to embryo cultures of D. stramonium, it caused an inhibition of about 

 70 per cent at a concentration of 10 p.p.m. but no significant inhibition 

 was observed by additions of the same concentration of desoxyri- 

 bonucleic acid. These investigations are being continued. 



SUGAR REQUIREMENTS AT DIFFERENT STAGES 



Rietsema et al. (1953), in a study of the sucrose requirements of 

 Datura stramonium embryos in vitro, incubated during 8 days, have 

 recorded the following observations. Different stages of development 

 require different minimal sucrose concentrations for growth: pre- 

 heart stages, 8-12 per cent; late heart stage, 4 per cent; and nearly 

 mature embryos grow even without sucrose. The optimal concen- 

 tration for hypocotyls is {±_) 8 per cent in the pre-heart stage and 

 decreases to 1 •0-0-5 per cent in the torpedo stage. Cotyledons and 

 hypocotyls have the same growth rate up to 5-8 mm. ; thereafter 

 hypocotyls alone continue growth. Roots develop when hypocotyls are 

 2-5-4 mm. long. Their optimal sucrose concentration is (±) 2 per cent. 

 Higher concentrations than 4 per cent decrease root formation. 

 Different embryo stages require different osmotic values of the medium. 

 The optimal osmotic value decreases as the embryo stage advances. If 

 the osmotic value is kept constant, all embryo stages respond in the 

 same way to changes in the sucrose concentration. 



CYTOGENETICAL AND PHYSIOLOGICAL STUDIES OF 

 ENDOSPERM AND EMBRYO 



In that the endosperm provides the medium in which the growth 

 of the zygote through to the fully organised embryo in the mature seed 

 takes place, and that the inception of both the embryo and endosperm 



