Polarity 145 



Only where polar differences occur, however, with the resulting morpho- 

 logical and physiological gradients from one end of the axis to the other, 

 can there develop the complex patterns characteristic of most living 

 organisms. 



The relation between these three aspects of polarity involves the prob- 

 lem of the origin of polarity itself. If they can be shown to form a pro- 

 gressive series, in phylogeny or ontogeny, this would indicate that polarity 

 may increase in complexity. In free-floating algal filaments (as in Spiro- 

 gyra) there is no evidence that the two poles are unlike. In filamentous 

 forms like Cladophora, however, where one end is attached to the sub- 

 stratum, each individual cell displays a polar character in its regeneration. 

 Here, and in many other cases where environmental factors are different 

 at the two ends of the axis (as in the Fucus egg), it appears that this 

 difference sets up an axial gradient in a system originally unpolarized, 

 which results in the polar difference. Child and his school regard all 

 polarity as having its origin in such environmentally induced gradients, 

 which determine both the direction of the axis and the difference be- 

 tween its poles. On the other hand, since instances of similar poles are 

 rare, it may be held that the two ends of every axis are fundamentally 

 unlike and that in cases where they seem alike the difference is merely 

 masked and difficult to demonstrate. If this view is correct, polarity may 

 be due to something quite different from a gradient and may be compa- 

 rable to, and perhaps result from, an inherent polar tendency, presumably 

 electrical in character. 



Whatever its origin, the direction of this bipolar axis is often continu- 

 ally changing but under definite control, and upon this fact depends the 

 orderly development of organic patterns. Thus in a three-faced apical cell 

 the polar axis must shift 120° between successive divisions. In more com- 

 plex meristems the planes of cell division are equally orderly, though less 

 evidently so. How such a system of changing polarities is controlled so 

 that growth in one direction is precisely related to that in another is a 

 part of the same problem of orderly development which the student of 

 morphogenesis so often meets. 



The ease with which polarity may be reversed in the simplest plants 

 suggests that even in more complex ones it is not irrevocably fixed by 

 genetic factors. Like any trait with a genetic basis, polarity is not a specific 

 characteristic but a specific reaction to a specific environment. The en- 

 vironmental factor may be external, such as the direction of light, or 

 internal, like the correlation between the axis of the young embryo and 

 that of the archegonium, but unless there is an environment to which the 

 organism can orient itself, the phenomena of polarity will rarely appear. 

 Sometimes this environmental reaction is determined early and is later 

 irreversible, as in cases where polarity becomes firmly fixed in the ferti- 



