Polarity 123 



ter. Pfeffer and Klebs have emphasized the probability that the cells of 

 the terminal growing points have no original polarity of their own, any 

 more than does an egg cell. In older parts a more stable polarization re- 

 sults from the influence of conditions in the environment. Vochting's idea 

 of the irreversibility of polarity in these higher plants has also been dis- 

 puted. Many investigations concerned with these problems cannot be 

 judged critically because of insufficient evidence, particularly as to 



anatomical facts. 



Polarity may be manifest in the transverse axis as well as in the 

 longitudinal one. This is evident structurally in the transversely polar 

 gradient often associated with regeneration. Thus Goebel (1908) found 

 that in half slices of the root of Dioscorea sinuata shoots grew out from 

 the central part of the axis and roots from the margin directly opposite 

 to this (Fig. 6-3). Transverse polarity is also manifest in the flow of 

 auxin in various tropisms (p. 384). The subject has been discussed in 

 detail by Borgstrom ( 1939 ) . 



Stem Cuttings. In stem cuttings, polar regeneration of shoots and roots 

 is clearly obvious in most higher plants, but there are considerable dif- 

 ferences between species. Polar behavior may be obscured in various 

 ways, as by the tendency of monocotyledons to form roots at nodes and 

 by the influence in many cases of the age of the cutting upon the forma- 

 tion of root primordia. The specific polar reactivity of tissues from which 

 buds and roots originate must be taken into account, as well as the fact 

 that a different complex of conditions may control each of the successive 

 processes in the development of these structures, such as the formation 

 of primordia, their growth, their final differentiation into roots and shoots, 

 or the formation of callus which may give rise to either roots or shoots. 



Various modifications of polar behavior in regeneration from stem 

 cuttings have been reported. Roots, for example, tend more character- 

 istically to be limited to one pole in their growth than do shoots. 

 Doposcheg-Uhlar (1911) observed this in Begonia, and Massart (1917) 

 studied 30 species of plants, some of which showed strongly polar re- 

 generation of both roots and shoots, some weakly polar regeneration, some 

 only root polarity, and some only shoot polarity. Root polarity was re- 

 lated to the growth habit of the plant, for species with pendant branches 

 rooted readily at their apical ends. 



Polar tendency is also expressed in the manner of callus formation 

 in cuttings, since in most cases callus tends to develop more vigorously 

 at the basal pole than at the apical. From the basal callus, roots are 

 usually formed, and shoots from the apical one. Simon (1908) noted 

 certain anatomical differences between apical and basal calluses and 

 made the observation that calluses from opposite poles may be made to 

 fuse but not calluses from the same pole. 



