450 Morphogenetic Factors 



be seen. Polarity is manifest in physiological activities as well as in struc- 

 ture. 



The universal presence of polar phenomena in plants suggests that 

 there is in living stuff an innate tendency toward polarization, although 

 most eggs and undifferentiated cells at first are unpolarized. This provides 

 a concrete point of attack on developmental problems. The suggestion is 

 obvious that organic polarities may be related to those evident in the 

 inorganic world, and especially to chemical and electrical ones, but this 

 has proved somewhat difficult to establish experimentally. Polarity has 

 been shown in some cases to be modified by specific factors in the en- 

 vironment. 



Not only are the two ends of the polar axis unlike but other differences 

 appear in graded series backward from each apex. These gradients are 

 evident in the rate of various metabolic processes and in the form and 

 structure of lateral organs arising successively at the growing point. Here 

 is a simple place to study the origin of differentiation. One school of 

 biologists regards these axial gradients as the most important factors in 

 development. 



The polar axis is not only a gradient axis but an axis of symmetry. In- 

 deed, the symmetrical arrangement of parts around it is a conspicuous 

 aspect of axiation. Organic symmetry, an expression of the basic regu- 

 larity in the arrangement of plant structures, is manifest at every level 

 from the internal structure of a cell to the configuration of a tree. It is 

 obviously an important element in the orderly formativeness that living 

 organisms display. 



Radial symmetry— the regular spiral distribution of lateral structures 

 around an axis— is best displayed in vertical plant axes where the stimuli 

 of gravity and light affect all sides of the axis almost equally. Under these 

 conditions, the symmetrical arrangement of the lateral organs seems to be 

 traceable to an inherent spirality in living stuff itself. This has as yet 

 received little experimental study, but its widespread occurrence in 

 phyllotaxy, the spiral grain of wood, the spiral character of the cell wall 

 and of protoplasmic streaming, and the spiral movements of the plant 

 body suggest that spirality is another basic fact in organization. 



Spirality is masked to a considerable extent in those axes which are 

 horizontal and thus exposed differentially to gravity and light, but the 

 dorsiventral symmetry of these structures provides good material for a 

 study of the interaction of polarity, spirality, and environmental factors 

 in the development of plant form. 



We may therefore recognize several components in the general phe- 

 nomenon of plant organization: polarity, differential gradients, symmetry, 

 and spirality. These seem to be distinct characteristics and may have dif- 

 ferent bases in protoplasm. They certainly can be investigated separately. 





