18 ' EMBRYOGENESIS IN PLANTS 



to form on the least illuminated side. (Rosenvinge, 1899; Kniep, 1907; 

 Whitaker and Lowrance, 1936). Hurd (1920) has shown that only the 

 shorter wave-lengths of the visible spectrum are effective (violet blue 

 and probably the shorter green). In experiments conducted in the dark, 

 in which eggs were subjected to a direct electric current, rhizoids 

 formed on the side towards the positive pole — at which negative ions 

 including those of auxin presumably accumulated (Lund, 1923). Where 

 eggs are assembled together in groups, the rhizoids point into the centre 

 of the group, Fig. 3. This effect of one cell on another acts over a 

 considerable distance— up to 0-3 mm. of sea water. (Rosenvinge, 1889; 

 Kniep, 1907; Hurd, 1920; Whitaker, 1931). Whitaker observed that 

 two eggs alone in normal sea water did not form rhizoids towards each 

 other, appreciable masses of eggs being necessary. Moreover, the 

 effect was found to be non-specific in that it was produced by un- 

 fertilised eggs of F. vesicuJosus on developing eggs of F. evanescens. 

 The presence of masses of eggs, releasing CO2 into the sea water, will 

 increase the hydrogen ion concentration; it was found that by acidifying 

 the water to pH 6-0, the 'group effect' in rhizoid formation was greatly 

 increased so that five or even two eggs together would show rhizoids 

 directed towards each other at distances up to four egg diameters. A 

 negative group effect was observed in alkaline sea water (Whitaker and 

 Lowrance, 1940). An egg responds to diffusion gradients from itself as 

 well as to the products of another egg (Whitaker, 1937), the rhizoid 

 forming on the side subjected to the greatest concentration of substances 

 diffusing from the egg. 



When individual eggs were placed between two pipettes, one 

 containing normal sea water at pH 8-0, and the other acidified to a 

 pH of 6-0 the rhizoid developed on the acid side (Whitaker, 1938) 

 (Fig. 3), If the acid pipette was at pH 5-6 the rhizoid developed on the 

 other side. Whitaker points out that gradient rather than actual pH 

 is the important consideration. 



DuBuy and Olson (1937) succeeded in extracting an auxin (un- 

 determined) from Fucus eggs and van Overbeek (1940) has shown that 

 auxin is present in Macrocystis. Olson and DuBuy (1937) found that 

 rhizoids form on the side supplied with the greatest concentration of 

 heteroauxin. Whitaker (1940) points out that the presence of auxin in 

 Fucus eggs does not prove that it is normally functional there, i.e. it 

 could be a waste product. 'The strong effect of acid on the Fucus egg 

 may well be due in part to its activating effect on auxin, but it appears 

 highly probable that so active an agent as the hydrogen ion would also 

 act on other steps in the rhizoid-forming process as well.' 



Fucus eggs respond to temperature gradients by forming the rhizoid 

 on the warmer side, provided the gradient across the egg does not 



