O 09 



Physiological and Anatomical Effects 117 



Cambium 



Pericycle 



Endodermis 



Ray and phloem parenchyma 



Pith 



Young phloem 



Xylem and mature phloem 



In this connection, it is interesting to observe that the seedling 

 stage is invariably highly sensitive to auxin applications (see chapter 

 XVI), and it is the stage with much the largest proportion of weakly 

 differentiated cells. Herbicide studies have shown many times over 

 that plants go through different stages of susceptibility to auxin herbi- 

 cides, and that the seedling is one of the most sensitive stages. The 

 young vegetative plant is less sensitive and also less meristematic than 

 the seedling. In the case of more mature plants of grain species, par- 

 ticularly at the stage of bolting when the rachis of flowers is being 

 lifted on a rapidly growing stem, a much higher sensitivity to auxin 

 herbicides is found (Blackman, 1950). It seems likely that the dif- 

 ferences in sensitivity to auxins with different stages of plant develop- 

 ment may be accounted for to a considerable extent on the basis of 

 the relative abundance of weakly differentiated cells in the plants at 

 the time of treatment. 



A striking example of anatomical changes brought about by auxin 

 treatment is shown in the sequence of pictures in figure 53. Stems 

 of decapitated bean plants were treated with lanolin paste containing 

 5,000 p. p.m. of 2,4-D, and sections taken at various time intervals after 

 application (Beal, 1946). The first picture (A) shows the normal 

 anatomical arrangement before treatment. The second picture (B) 

 shows the appearance after 24 hours, when meristematic activity has 

 already started in the endodermis and phloem parenchyma, and activ- 

 ity is beginning to appear in the cambium, ray parenchyma and peri- 

 cycle. The third picture (C), at 33 hours, shows widespread meri- 

 stematic activity in all tissues from the endodermis to the cambium, 

 including the parenchymatous areas between the vascular bundles. 

 The fourth picture (D) shows at 72 hours the immense spread of 

 undifferentiated callus-like tissues through the stem, associated with 

 gross swelling of the stem. This stage is followed by the differentiation 

 of root primordia, which become scattered through the mass. It can 

 be seen in this sequence that the weakly differentiated endodermis, 

 cambium and parenchymas Averc the first tissues to respond. With 



