Physiological and Anatomical Effects 119 



weaker applications these might have been the only ones to show 

 meristematic activity. 



The tissues which respond to the greatest degree vary somewhat 

 with the age of the individual plant part. Thus pea stems proliferate 

 most actively and form root primordia mostly from endodermis in 

 younger stems, and mostly from pericycle in older stems (Falser, 1942). 



Large differences in response of various tissues to different auxins 

 have been observed. One auxin may characteristically cause the prolif- 

 eration of the phloem; another one may primarily affect the endoder- 

 mis. Some examples of the gross differences in response obtained with 

 different auxins are discussed in the section in this chapter on leaf 

 effects. 



The mode of transport of the auxin applied and its ultimate dis- 

 tribution in the plant undoubtedly strongly influence the type of 

 anatomical effects obtained. For example, if the auxin is transported 

 primarily in the phloem, as it frequently is, then the phloem is prone 

 to be one of the first tissues to show anatomical response. Much less 

 transport probably occurs in the cambium. Having less auxin there, 

 the cambium may not be the first tissue to undergo proliferation even 

 though its sensitivity to auxin is very great. 



Hamner and Kraus (1937) have shown that a uniform ring of 

 auxin paste around a stem produced much greater amounts of callus 

 in zones overlying the vascular bundles than in zones between 

 bundles. A parallel observation has been made on roots (Wilde, 1951), 

 where active proliferation of pericycle tissues in young roots was 

 heavily concentrated over the vascular strands with much less prolif- 

 eration between the strands. 



The ultimate distribution of the growth regulator in the plant 

 will be determined not only by the locus of transport, but by the 

 transportability of the molecule itself and the general rate of trans- 

 location of other materials occurring in the plant. 



It seems safe to assume that some environmental influences may 

 alter the anatomical effects of auxins, especially since they will change 

 the amount of translocation and otherwise adjust the constituents of 

 the plant. 



Stem Effects 



Among the many studies of changes in stem anatomy following 

 growth regulator applications, those by Beal (1945-46) might be 

 selected as showing particularly well the differences in anatomical 

 response owing to differences in the translocation of the applied 

 auxin. For example, he found that the application of 2,4-D in lanolin 



