106 Fundamentals of Auxin Action 



roots. Cuttings, particularly of woody plants, connnonly grow callus 

 followed by root formation at the physiological base of the cutting. 

 These events can be greatly encouraged by the applications of rather 

 weak auxins, the one in most common use being indolebutyric acid. 

 The fact that auxin treatment is effective in bringing about root for- 

 mation l)ut not bud formation becomes understandable. 



Morphological studies of the development of calltis show that 

 whatever the tissue of origin, a successive series of cell divisions occins 

 which typically ends in a fan-shaped arrangement of cells shaped like 

 packed bubbles without any real differentiation of the usual cell types. 

 This represents a rather complete stage of dedifferentiation of individ- 

 ual cells into masses of dividing cells, each one closely comparable to 

 its neighbors and showing no inclination to differentiate into a mature 

 type of tissue such as xylem, phloem, or even parenchyma. 



The first stage in differentiation from this undifferentiated callus 

 is the formation of a meristem. Sterling (1951) has shown in some 

 detail that clusters of cells on the surface or stibadjacent to the surface 

 first become differentiated into a tunica-like arrangement of rectangu- 

 lar cells and from this first step in differentiation a compact meristem 

 soon emerges, being either root primordium or a bud apex depending 

 on the biochemical factors mentioned above. 



It may not be out of place briefly to consider the phenomenon of 

 abscission at this point. The abscission layer in the petiole of a leaf or 

 the pedicel of a flower or fruit is generally a meristematic zone which 

 develops, as far as we know, when the auxin supply from the plant 

 organ becomes low. Thus we may consider that the abscission of these 

 plant parts is caused by the development of a meristematic tissue when 

 the relative atixin content is low. The prevention of abscission of plant 

 parts by the atixin formed in them then appears to be a natural con- 

 trol of meristematic differentiation by auxin. 



Bud formation is another type of organ differentiation which is 

 strongly altered by auxin concentrations as has been discussed already. 

 It is interesting to notice that while buds are relatively easy to induce 

 in stem material, they are quite difficult to induce in most root cut- 

 tings. Consequently, plant propagation techniques practically never 

 utilize root cuttings. Although they will form new roots easily, buds 

 will be produced readily in only a few species. In view of the high 

 auxin sensitivity of roots and the demonstration by Skoog and Tsui 

 that the effective ratio of auxin to adenine must be low for the forma- 

 tion of buds, it is perhaps not surprising that such a ratio is seldom 

 attained in roots. Means of bringing that about artificially are described 

 in chapter X. 



