BOTANY AND MORPHOGENESIS 121 



nal bud sometimes inhibits a lateral one; or compensatory, as where removal 

 of one part results in abnormally large growth of another. The correlations 

 evident in the development of specific forms, where physiological factors are 

 not directly involved, are obviously under genetic control. Allometric analyses 

 of the development of such forms have been fruitful although as yet little 

 more than descriptive. The term "growth correlation" used to be thought of 

 as a convenient catchall for unsolved problems in development, but the dis- 

 covery of various mechanisms involved in it, especially among plants, has 

 begun to bring order out of its confusion. 



Polarity is also a conspicuous feature in development. Most organisms are 

 built around a polar axis, the two ends of which are unlike. This is especially 

 conspicuous in plants. Polarity here is manifest in various ways. One of these 

 is in the basic difference between the root pole and the shoot pole and the 

 structures each produces. Polarity is evident even in isolated pieces of a plant 

 axis, as shown by the polar regeneration of roots and shoots in pieces of young 

 stems and of fleshy roots. In some of the simple plants this polar character 

 has been shown to be present even in individual cells as they regenerate, a 

 fact which the firmness of plant tissues and the fixed position of their cells 

 make it possible to prove more easily than in animals. 



There must be a physiological polarity underneath that which is visible 

 in structure. The simplest and best-known example of this is the polar flow 

 of auxin, which is almost always from the apical end of the axis basipetally 

 and is usually difficult to reverse. The actual mechanics of this one-way flow 

 is unknown, but it seems not to be electrical in character. Bioelectric factors 

 are evidently involved in some cases of physiological polarity, however. Since 

 auxin tends to accumulate in the lower portions of the axis and since a rela- 

 tively high concentration favors root development and a lower one shoot 

 development, this fact may account for the polar nature of regeneration in 

 these structures. Transverse as well as longitudinal polarities have been 

 demonstrated. All this emphasizes the morphogenetic importance of the fac- 

 tors which determine the direction of movement of growth substances and 

 nutrient materials in the plant. Perhaps a complex pattern of specific polari- 

 ties is the basis for the patterns of tissues and organs. However this may be, 

 the actual nature of polarity itself is obviously a major problem. A study of 

 it in its relatively simple manifestation in plant axes has been very fruitful 

 and seems to be the place where this basic problem may be attacked most 

 hopefully. 



Symmetry is another morphogenetic phenomenon of the first importance. 

 Here, again, plant material makes a unique contribution, since plant axes 

 are prevailingly vertical instead of horizontal, as in animals. The radial sym- 

 metry of leaves spirally arranged around this axis, and of flowers and floral 

 parts, shows simple mathematical relationships. There is some difference of 

 opinion as to how these phyllotactic patterns originate, but they are prob- 



