118 The Phenomena of Morphogenesis 



Polarity is involved in many morphogenetic phenomena, and it will 

 necessarily be referred to repeatedly in other chapters. Thus symmetry 

 is the orderly distribution of structures in relation to a polar axis. Polar 

 differences are the simplest aspect of differentiation. Regeneration is in 

 most cases a polar process. Form results from a pattern of polarities set 

 up in the developing plant. Polarity may be regarded as the framework, 

 so to speak, on which organic form is built. 



The polar behavior of plants has long attracted the attention of 

 morphologists and physiologists, from whose work a great body of knowl- 

 edge has accumulated. Theophrastus and other ancient writers described 

 the abnormal behavior of plants grown in an inverted position. There are 

 a number of other early observations, especially those on regeneration 

 following the girdling of trees, by Agricola, Hales, and Duhamel du 

 Monceau. The term polarity was used by Allman in 1864 in connection 

 with phenomena of animal regeneration and is now generally employed 

 by students of morphogenesis in both botany and zoology. Vochting made 

 extensive studies of polarity in plants in its relation to problems of re- 

 generation, growth, and differentiation (1878, 1908, 1918). Important 

 botanical work was also done by Goebel (1908), Janse (1906), Loeb 

 (1924), and others. The theoretical aspects of polarity have been ex- 

 tensively discussed not only by Vochting but by Sachs (1882), Pfeffer 

 (1900-1906), Klebs (1903, 1904, 1913), Winkler (1900, 1933), Went and 

 Thimann (1937), Lund (1947), Bunning (1952b), and others. Polarity in 

 animals has been studied by many workers, notably Driesch, Roux, 

 Morgan, and Harrison. Reviews of the field of plant polarity or parts 

 of it have been written by Bloch (1943a), Gautheret (1944), and 

 Bunning ( 1958 ) . Polarity is important not only for theoretical problems 

 of plant development but for many practices of horticulture and vege- 

 tative propagation (Priestley and Swingle, 1929). 



The establishment of a morphological axis in which the two ends are 

 different and along which there is a gradation from one pole to the 

 other may be looked upon as the first step in the process of differentia- 

 tion, an important aspect of morphogenesis. The expression of polarity 

 differs considerably in different plants and under different environmental 

 conditions and is thus open to a wide range of experimental investigation. 

 This most conspicuous aspect of organic form will probably not be fully 

 understood until the mechanism of orderly and correlated growth control 

 is discovered. As a relatively simple manifestation of form, however, it 

 provides a useful point of attack on morphogenetic problems. 



Polar behavior in plants presents many problems. How far is it an in- 

 herited character, potentially present from the beginning of development, 

 and how far induced by the action of various environmental factors or by 



