142 The Phenomena of Morphogenesis 



involved in this polar flow would doubtless contribute to the solution of 

 many problems in physiology and development. 



POLARITY AND DEVELOPMENTAL PATTERN 



The chief significance of polarity for students of morphogenesis lies in 

 the fact that it is the simplest expression of the general phenomenon of 

 organic pattern. These patterns, which are exhibited in such profusion in 

 the bodies of animals and plants, are each built around a polar axis 

 which provides, so to speak, the theme or foundation upon which the 

 whole develops. This, of course, is by no means the only expression of 

 polarity in the plant. In a tree, for example, not only the main trunk but 

 the many branches growing out from it may each have a polar axis of its 

 own. The frequency and size of these branches and the angles which 

 they make with the trunk produce the characteristic pattern of the tree's 

 crown, one which is almost as specific as the pattern of its leaf. The char- 

 acter of this crown is due to the dominance of certain buds or branches 

 over others and thus to a controlled localization of growth and a balance 

 between the various axes of the tree. This, in turn, seems to be governed 

 by a specific polar pattern of auxin distribution. It seems probable that 

 the form and development of a leaf, which involve a pattern of major and 

 minor vein polarities, have a similar basis. In such cases as these, organic 

 form appears to be the expression of a series of interrelated polar axes. 



Such a condition probably occurs also in forms in which the organic 

 pattern is related to the polarity of individual cells. This is well shown 

 in the development of shoots that grow by a large apical cell. Here the 

 growth and differentiation of the axis are clearly associated with a precise 

 series of divisions in various planes, both of the apical cell and of those 

 cut off from it (p. 58). This pattern of diverse cell polarities is less easy to 

 trace in other meristematic regions but is evidently operating there as 

 well. In such a structure as the growing primordium of a young ovary, 

 cell divisions are very abundant but occur in every direction, as though 

 the planes of division were at random. That such divisions are all part of 

 a definite organic pattern, however, is shown by the fact that the structure 

 in which they occur shows a regular and progressive development toward 

 its specific form. Each plane of division, presumably determined by the 

 orientation of the cytoplasm, is related to the complex pattern of diverse 

 polarities of which it forms a part (Sinnott, 1944). 



The polar phenomena of coenocytes and other evidence support the 

 contention that the basic fact in polarity is the orientation and polar be- 

 havior of the cytoplasm. Where this is confined within cell walls, more 

 complex and stable patterns may be produced, but the fundamental prob- 

 lem everywhere seems to be the development of polar patterns in the 



