CHAPTER 6 



Polarity 



In plant development, growth does not proceed at random to the produc- 

 tion of a formless mass of living stuff but is an orderly process that gives 

 rise to specific three-dimensional forms of organ or body. The various cor- 

 relations described in the preceding chapter are manifestations of this 

 formative control, which knits the developing organism together so that 

 growth in one region or dimension is related to growth in the others and 

 the plant thus becomes an integrated individual. A notable feature of 

 these bodily forms of plants ( and animals ) is the presence in them of an 

 axis which establishes a longitudinal dimension for organ or organism. 

 Along this axis, and symmetrically with reference to it, the lateral struc- 

 tures develop. The two ends or poles of the axis are usually different 

 both as to structure and physiological activity. Thus a typical vascular 

 plant has a major axis with the root at one end and the shoot at the 

 other and with lateral appendages— leaves, branches, or lateral roots- 

 disposed symmetrically around it. Growth is usually more rapid parallel 

 to the axis than at right angles to it, so that an elongate form results, 

 though this is by no means always the case. Single organs such as leaves, 

 flowers, and fruits also show axiate patterns, as do the bodies of lower 

 plants. These patterns appear very early in development as the result 

 of differences in growth or in planes of cell division. This characteristic 

 orientation of organisms, which is typically bipolar and axiate, is termed 

 polarity. 



Polarity may manifest itself in many ways. The structures at the two 

 ends of an axis are unlike, as in the case of root and shoot, "stem end" 

 and "blossom end" of fruits, and petiole and blade of leaves. In re- 

 generation, the organs formed at one end are usually different from those 

 formed at the other. Cells and tissues may show polar behavior in graft- 

 ing experiments. The transportation of certain substances may take place 

 in one direction along the axis but not in the other, thus manifesting 

 polarity in physiological activity. Both in structure and in function there 

 are gradients of all sorts. Individual cells show polar behavior in plane 

 of division and in the different character of their two daughter cells. 



116 



