16 EMBRYOGENESIS IN PLANTS 



acropetally. Great differences in transport velocity have also been 

 found among various other growth-promoting substances. 



White (1934) has expressed the view that the methods of tissue 

 culture may be used in the investigation of polarity. He points out that 

 while the formation of specific organs may be due to specific substances, 

 the process may also be affected by concentration gradients of various 

 kinds and 'hence by polar (physical) as well as chemical relationships.' 

 Possible gradients of pH, or of redox, electrostatic or hydrostatic 

 potential, may be involved in different instances ; but the primary cause 

 of polarity is still unknown. Gautheret (1945), also using tissue culture 

 data, has advanced a theory of polarity based on experimental 

 observations. He notes as a striking fact that the direction in which 

 buds, formed on the foliar end, or shoot face, of segments of endive 

 root, exercise their effect on root development and inhibition of lateral 

 buds, corresponds with the direction of circulation of auxins. By 

 assuming the existence of a polarised circulation he is able to offer 

 various interesting ideas as to how undifferentiated tissue, growing in 

 culture, becomes organised into an axial structure with an apical bud, 

 leaves and roots ; but the basic problem of polarity still remains un- 

 solved and, in his view, must be sought, not at the tissue level of 

 development, but at the cellular level. 



Polarity in Fucus Eggs} Whitaker (1940) has discussed the results 

 of a long series of experiments (Whitaker, 1931-1940) on the polarity 

 in the eggs of the brown algae, in particular of the genus Fucus, Figs. 

 3, 4. In these investigations he has attempted to answer the question: 

 In the spherical, free-floating zygote of Fucus, what factors determine 

 the axis of growth and differentiation? He has expressed the view 

 that: 'The effects on the living protoplasm of physical and of chemical 

 factors become largely indistinguishable if the physiological analysis 

 can be carried far enough.' Any physical change inevitably affects 

 biochemical processes ; moreover, the distinction between physical and 

 chemical properties 'largely disappears at the molecular and sub- 

 molecular levels at which we believe the ultimate, even if mostly 

 unidentified, processes of differentiation and growth take place.' 



The fertihsed egg of Fucus is a spherical and relatively undifferen- 

 tiated body. Growth and differentiation, i.e. a quantitative and a 

 qualitative change, take place simultaneously, the first visible manifesta- 

 tion being the appearance at one point of a rhizoidal outgrowth. The 

 first cell division takes place in the plane at right angles to the emerging 

 rhizoid. The factors which determine the position of this rhizoid 

 therefore determine the polarity of the developing germ. Polarity is 



1 In the experiments of Whitaker and others the term 'eggs' usually refers to zygotes. Where 

 unfertilised eggs are Indicated, they are so described. 



