Growth Substances 



399 



is even to be found in the algae, for von Denffer and Hustede (1955) 

 were able to shift the sexual phase of Vaucheria sessilis to the vegetative 

 one by treatment with indoleacetic acid (Fig. 18-19). 



Sex Determination. Both in the determination of the sex of individual 

 plants and in the development of the sex organs, growth substances of 

 various sorts seem to be effective. 



In the dioecious species Cannabis sativa (hemp) it is possible to dis- 

 tinguish genetically male from genetically female plants before they 

 flower. Heslop-Harrison (1956) grew plants under controlled photo- 

 periodic conditions and during the period of differentiation of flower 

 buds applied naphthaleneacetic acid to leaves at the third and fourth 

 nodes. In genetically male plants, female flowers were produced, sug- 



-10 -11 . 

 m m 



JES 9/an J 



Fig. 18-19. Effect of different concentra- Fig. 18-20. Young prothallia of Pteridium 

 tions of IAA on the proliferation of an- aquilinum 11 days after spore germina- 

 theridial primordia in Vaucheria. (From tion. A, grown on agar to which a water 

 von Denffer and Hustede. ) extract of prothallia was added. B, grown 



on ordinary agar. The extract stimulates 

 early development of antheridia. (From 

 Dbpp. ) 



gesting that sexuality is determined by the concentration of native auxin 

 during the period of primordium differentiation and that femaleness is 

 associated with a relatively high auxin level. In Mercurialis ambigua 

 he found ( 1957) that carbon monoxide much reduced the number of male 

 flowers in genetically monoecious types, presumably by its effect on 

 auxin. Laibach and Kribben (1951) painted the lower surfaces of the 

 leaves of cucumber, a monoecious plant, with naphthaleneacetic acid 

 in lanolin and caused an increase in the proportion of female flowers, 

 sometimes altogether suppressing the differentiation of males. Indole- 

 acetic acid they found to be less effective and 2,4-D more so. Extending 

 his work to other plants, Laibach concludes (1953) that, in general, 

 female flowers or female parts of flowers tend to differentiate under 



