424 PATTERNS AND PROBLEMS OF DEVELOPMENT 



at the tip. The other cell gives rise to the thallus; and, as this develops, 

 a gradient, with high end apical gradually appears in it. It has long been 

 known that polarity of this egg can be determined by light, that is, by 

 differential illumination, the rhizoid developing on the side away from 

 the light, with the first division plane transverse to the direction of illumi- 

 nation.^^ More recently a certain range of wave length toward the blue- 

 violet has been shown to be chiefly effective (Hurd, 1919, 1920). With 

 repeated (e.g., hourly) change of position with respect to direction of in- 

 cident sunlight, the axis indicated by outgrowth of rhizoid and the divi- 

 sion plane may form any angle with each other (Fig. 143, B-D). From 

 changes of 180° in position bipolar forms frequently result, either with 

 rhizoids at or near opposite poles (Fig. 143, E-G) or with equal division 

 and no rhizoids, like Figure 143, i^ (Child, unpublished). These varia- 

 tions suggest that rhizoid outgrowth and cell division require different 

 periods or intensities of illumination for determination of the usual rela- 

 tion, but further experiment on this point is desirable. 



Polarity of this egg may also be determined by electric current, the 

 rhizoid developing on the side toward the anode (Lund, 19236). When 

 several eggs lie close together, there is mutual determination of polarity, 

 the rhizoids arising toward the center of the group, thus indicating a 

 chemical differential dependent on presence of the eggs. It has been fur- 

 ther shown that increase in H-ion concentration increases this group effect 

 and that a H-ion gradient induces rhizoid development on the more acid 

 side, up to a certain concentration, and above that on the less acid side. 

 However, even with removal of excess CO2 from acidified sea water the 

 possibility that it is concerned in this effect is not entirely excluded.^' In 

 another line of experiment it was found that the rhizoid tends to form on 

 that side of the egg to which a sufficient concentration of the potassium 

 salt of indole acetic acid (heteroauxin) is applied, and in the light of these 

 results it was suggested that certain agents which determine polarity in 

 this egg may do so by influencing in some way the activity of auxin in 

 the egg.''° Whitaker points out that this hypothesis will account for most 

 effects of increased H-ion concentration, since acidity increases auxin ac- 

 tivity. On this basis effects of high H-ion concentration result from in- 

 crease in auxin activity to the point at which it becomes inhibitory. 



^* Farmer and Williams, 1898; Winkler, igoob; Kiister, 1906; Kniep, 1907. The first 

 division plane and the polarity of the Equiselum spore show a similar relation to direction of 

 incident light (Stahl, 1885). 



^"Whitaker, 1935, 1937a, 1938a; Whitaker and Lowrance, 1937. 



i" Du Buy and Olson, 1937; Olson and Du Buy, 1937. 



