POLARITY 623 



a theory the probability seems very great that this type of growth correla- 

 tion is also to be explained in terms of a hormonal mechanism. 



Polarity. — Many growth correlations are polar; that is, the two ends of 

 a growing axis exhibit a marked dimorphism in development. The most 

 familiar example of polarity in plants is that shown by cuttings, in which roots 

 develop from the basal end and shoots from the apical end. Even if such cut- 

 tings are inverted and kept in a moist atmosphere roots will usually develop 

 only from the morphologically basal end and shoots only from the morpho- 

 logically apical end. In general, it is difficult to modify the inherent polarity 

 of plant organs, although some exceptions to this statement are known. 



While the obvious manifestations of polarity are morphological, basically 

 all such phenomena depend upon a physiological mechanism. IVIany of the 

 polar phenomena of plants are probably due to the polar transport of auxins 

 or other hormones. The polarity of cuttings (Chap. XXXII) apparently 

 can be explained largely, if not entirely, on a hormonal basis. 



In addition to morphological polarities, and polarities in the distribution 

 of certain compounds, plants also exhibit electrical polarities. The apices 

 of the stems, hypocotjls, and coleoptiles of a number of herbaceous species 

 have been shown to be electronegative relative to more basal portions (Clark, 

 1937)- In larger plants, such as trees, the distribution of gradients of elec- 

 trical potential are more complex (Lund, 1931). Apparently each individual 

 cell in a plant is electrically polarized and acts like a tiny battery. The 

 electrical potentials occurring in plant tissues are summation effects of the 

 potentials of individual cells which may act either in series or in parallel 

 (Rosene, I935)- 



As a result of differences in electrical potential from one part of a plant 

 to another electric currents flow continuously along certain circuits in the 

 plant, and it has been suggested that these may serve as a mechanism of 

 correlation. Electrical energy may thus be transferred from one cell to 

 another, influencing the processes or development of the recipient cell. Al- 

 though there is no doubt that polarity potentials exist in plants, the evidence 

 that they are the basis of a correlation mechanism is as yet very insubstantial. 

 On the other hand, it is possible that the differences in electrical potentials 

 known to exist in plants are purely secondary phenomena which bear no casual 

 relation to the correlative behavior of plants. 



Discussion Questions 



I. What environmental conditions lead to an increase in the shoot-root ratio? 

 Which will lead to a decrease? Explain the probable mechanism of the 

 action of each factor. 



