124 Fundamentals of Auxin Action 



many species the marginal meristems of these leaves tend to develop 

 normally, so that a margin or frill of green tissues appears as in the 

 center leaf. In bean leaves this frill extends around the entire leaf 

 (Eames, 1951). 



The number of leaves showing extreme distortions apparently 

 depends upon the degree of persistence of the active auxin in the meri- 

 stems. In beans and peppermint, for example, only two or three leaves 

 will be so distorted, whereas in cotton plants the extreme distortion 

 may affect as many as eight leaves (Mcllrath and Ergle, 1953). Leaves 

 which develop after the influence of the auxin has been largely dis- 

 persed show beginnings of normal development of islets of mesophyll 

 at the leaf bases but still suffer a persistent distortion of the apices. 

 Finally, leaves which develop entirely after the effect of the auxin 

 has been lost show completely normal form again, as seen in the right 

 leaf in the figure. 



Auxin treatments can engender a wide variety of leaf deformities, 

 depending upon the specific tissues influenced by the treatment. For 

 example, epidermal development may be retarded so that the leaf 

 surface puckers (Burton, 1947). Characteristically the spongy meso- 

 phyll tissue divides excessively, so that the intercellular spaces of the 

 leaves fill up (Watson, 1948). A great reduction in the photosynthetic 

 capacity of the leaves is brought about in addition by the reduced 

 frequency of chloroplasts as well as by the shrinkage in leaf size. High 

 concentrations of 2,4-D can effectively stop photosynthesis in bean 

 leaves within 24 hours, as a result of the collapse of cells and prolifera- 

 tion of replacement tissue in the spongy mesophyll of the leaf (Lou- 

 stalot and Muzik, 1953). 



Root Effects 



The capacity of auxins, both endogenous and applied, to induce 

 root formation has been recognized since the earliest days of auxin 

 research. The initiation of adventitious roots on stems and of lateral 

 roots on roots is one of the main morphological responses commonly 

 observed following treatment. The structure of lateral root primordia 

 led Thimann (1936) to suggest that the origin of nodules on legumi- 

 nous plant roots may be partly a residt of the high auxin production 

 known to occur in Rhizobiiim infections of roots. 



In some cases auxin applications may bring about such abundant 

 lateral root formation that a fasciated condition occurs, with whole 

 clusters of root primordia in longitudinal rows down the sides of the 

 young growing roots. A striking example of this type of response in 

 the bean root is shown in figure 56 (Wilde, 1951). These fasciated rows 



