Polarity and gradients in plants: A survey

Abstract

The growth of a plant is very rarely unorganized, resulting in a formless aggregate of living material; it is as a rule an orderly process which creates specific, mostly three-dimensional forms, and the plant thus develops as an integrated individual with typically shaped organs and body. In most plants (and animals) both the body as a whole and the individual organs possess an axis which establishes a longitudinal dimension. The different structures develop along this dimension and symmetrically with reference to the axis (or to a median. longitudinal plane). A gradation in character may be found along the axis, and its two ends or poles are usually different, both as to morphological character and physiological behavior. Thus in the fertilized egg of the alga Fucus the plane of the first division sets apart the anchoring portion and the rhizoid from the free thallus, and in a typical vascular plant there is a major axis with the root at one end, the shoot at the other, and lateral appendages — leaves, branches and secondary roots — symmetrically arranged around it. Growth is generally more active parallel to this axis than at right angles to it, resulting in an elongate structure, though exceptions to this are not infrequent. Individual organs such as leaves, flowers and fruits also show axiate patterns. Cells are also often clearly axiate, and so are the bodies of most lower plants. This bipolar, axiate character of organisms and their parts has been termed polarity.