Polarity 139 



than to gravity is responsible for this reversal. Steinecke (1925) has 

 shown that in these inverted plants there is a movement of the cyto- 

 plasm that was originally in the upper portion into the base, and vice 

 versa. The easy reversal in such plants seems to be related to the fact 

 that the cytoplasm can move readily throughout the whole body. The 

 cellular organization of higher plants may contribute to the more fixed 

 polarity that they display as well as to their higher degree of differen- 

 tiation. 



Caulerpa has a more complex structure than Bryopsis, for it possesses 

 a horizontal "rhizome" from which "leaves" grow out above and rhizoids 

 below. The leaves are negatively geotropic, the rhizoids positively so, 

 and the rhizome is diageotropic. Regeneration in this plant has been 

 studied by many workers (Wakker, 1886; Janse, 1906 and 1910; Dostal, 

 1926 and 1929; Zimmermann, 1929; and others ) . Its polar phenomena are 

 rather complex. Zimmermann found that gravity determines the dorsi- 

 ventrality of the rhizome and that this can be reversed. He also observed 

 that in each portion of a cut leaf new rhizoids are formed below new 

 leaves. An inverted leaf with its tip buried will produce leaves at its 

 original base and rhizoids at its original apex, as in Bryopsis. Janse has 

 shown, however, that rhizoids normally appear chiefly at the apical por- 

 tions of cut leaves, and Dostal finds that, although regeneration is polar 

 in young leaves, the new organs may be distributed over the entire sur- 

 face of older ones. Polarity in Caulerpa is less stable than in higher plants 

 and this, again, is probably because of the ease with which cytoplasmic 

 movement may take place. 



PHYSIOLOGICAL MANIFESTATIONS OF POLARITY 



Differences in the external or internal structure of the plant body are 

 almost invariably accompanied by physiological differences, though the 

 latter are usually more difficult to demonstrate. Among these are the uni- 

 directional flow commonly shown by auxin and often by other substances; 

 the differences in bioelectric potential which can be demonstrated be- 

 tween different parts of the plant; and the many examples of physiological 

 gradients in the plant body— in pH, rate of respiration, osmotic concentra- 

 tion, auxin concentration, and others. These are doubtless related to 

 visible morphological polarities but the character of the relationship is 

 obscure. Whether such physiological polarities control the morphological 

 ones or whether both are determined by more deeply seated morpho- 

 genetic factors in the living material, which are physiological only in the 

 broadest sense, is not known. 



Electrical polarities are found in many organs and in the plant body as 

 a whole. Unfortunately, any discussion of the significance of electrical 



