50 EMBRYOGENESIS IN PLANTS 



rhizoid formation. At 10-20 per cent salinity the eggs burst and cytolyse 

 and in concentrated sea water development is inhibited but this may be 

 reversible (Whitaker and Clancy, 1937). 



After fertilisation there is a relatively long delay — 12-24 hours, 

 average 18 hours — before rhizoid-formation begins in eggs kept in 

 darkness at 15°C. The first cell wall, separating the rhizoid from the 

 spherical body of the zygote, is visible after about 25 hours. In con- 

 sidering how various experimental treatments affect the egg of Fucus, 

 Whitaker (1940) points out that the physiological and morphogenetic 

 analyses require consideration of both the metabolic and biophysical 

 aspects, which are, in fact, inseparable. 



Indications of metabolic and biophysical changes in the Fucus 

 zygote are afforded by its response to light. In fertilised eggs reared in 

 the dark at 15°C, the susceptibility to light (as indicated by the induction 

 of rhizoids on the non-illuminated side) begins 3-4 hours after fertilisa- 

 tion and ends at about 16-18 hours, the maximum response being at 

 8 hours. But this period of maximum susceptibility to light precedes 

 the rhizoidal outgrowth by several hours: 8-10 hours elapse before 50 

 per cent of a population in the dark develop protuberances. (Whitaker 

 and Lowrance, 1936). From such data it may be inferred that only 

 when various metabolic and other changes have taken place is the 

 zygote competent to react to certain morphogenetic stimuli. 



The effect of centrifuging the eggs of Fucus and other algae has 

 already been mentioned (p. 19). Whitaker (1940) has shown that when 

 fertilised eggs of Fucus furcatus are stratified by ultra-centrifuging, 

 most of the eggs remain spherical and the stratifications usually persist 

 during rhizoid formation and until the first transverse wall appears. 

 In normal sea water, with a pH of 7-8-8-1, in the dark, the rhizoid grows 

 out at the centrifugal pole ; but if the sea water is acidified to a pH of 

 6-0, the developmental response is reversed, the rhizoid being formed in 

 the centripetal half (Fig. 4). If the pH is between 6-0 and 8-0 the 

 response of a population of eggs is intermediary, the rhizoids arising 

 at random with respect to the stratification. To explain these experi- 

 mental data Whitaker points out that the rhizoid typically forms at the 

 more acid end where there is an associated accumulation of auxin, and 

 he has suggested tentatively that the centripetal or lipoid pole (at which 

 there is an accumulation described as an oil cap) has less buffer capacity 

 than the centrifugal pole, and that its pH is therefore more affected by 

 the pH of the medium. An internal pH gradient would result unless the 

 pH of the medium and the protoplasm were identical, and its direction 

 would be reversed in a medium at pH 8-0 as compared with pH 6-0. 

 In these experiments Whitaker also observed that 'a group effect' i.e. 

 the effect of diffusates from adjacent eggs, may be superimposed on the 



