GENERAL CONCLUSIONS 241 



discussed. In this large uninuclear siphonaceous cell, the 

 apical end of any isolated middle section will regenerate 

 apical organs; the basal end, on the other hand, will regen- 

 erate rhizoids. Sections containing the nucleus regenerate 

 rapidly and completely, but without the nucleus regenera- 

 tion is only complete if the section is fairly old and has 

 therefore been under the influence of the nucleus for some 

 time. Hammerling interprets his results as due to polar 

 movement in opposite directions of two substances whose 

 production is controlled by the nucleus, one apex-forming 

 and the other base-forming. The percentage of regenerates, 

 and the rate at which they form, are both proportional to 

 the size of the isolated piece, and therefore, as he concludes, 

 to the amounts of organ-forming substances present. The 

 conclusions are supported by numerous additional experi- 

 ments, but it must nevertheless be emphasized that as yet 

 no direct e\adence for the postulated organ-forming sub- 

 stances has been brought forward. 



A comparable case is that of Griffithsia hornetiana. By 

 passing an electric current through this alga, Schechter 

 (1934) could induce rhizoids to form at the side facing the 

 positive pole. The number of rhizoids was increased by in- 

 creasing the current density. Schechter suggests that the 

 formation of rhizoids is induced by some material inside the 

 cell, which moves in an electric field. Centrifugal forces 

 have a comparable effect (Schechter, 1935) ; shoots appear on 

 the side where the hea\der matter collects. Similar electrical 

 control of polarity was reported for the zygotes of Fucus 

 by Lund (1923); the rhizoids grow out towards the positive 

 pole. Thus, the inherent polarity in lower plants, or single 

 cells, may be affected by electrophoresis of organ-forming 

 materials resulting from applied potentials. This is not true 

 for higher plants. 



To explain differences between types of regenerates, and 

 between growth rates at different points, the concept of 

 ''gradients" or ''fields" has been introduced. So long as 

 the gradient is one of something intangible it is not open to 



