404 M or pho genetic Factors 



between cells in their response to wounding or wound hormones. Such 

 cells evidently differ in character and reactivity. Thus the root pericycle 

 and the vascular cambium respond to injuries by the production of 

 wound tissue much more readily than do adjacent cells of the ground 

 parenchyma. Auxin may be one factor which produces such specific 

 reactivity. In Populus balsamifera, Brown (1937) found that the cam- 

 bium was stimulated to active growth both by wound hormones released 

 from dead cells and by a substance, presumably auxin, coming from buds 

 and leaves distal to the wound. Application of auxin above the wound 

 considerably increases a local wound reaction (Brown and Cormack, 

 1937). Other observations confirm this (Bloch, 1941). 



GROWTH SUBSTANCES AND INTERNAL DIFFERENTIATION 



Numerous instances have been reported where specific changes in in- 

 ternal structure are related to the action of auxin or one of the synthetic 

 growth substances. 



In the regeneration of buds on the decapitated hypocotyl of flax (p. 

 245), Link and Eggers (19466) found that this was largely prevented if 

 indoleacetic acid in lanolin was applied to the cut surface. Even the 

 transverse division of epidermal cells, the first visible step in bud pri- 

 mordium differentiation, was usually inhibited. 



Nysterakis and Quintin (1955) report that application of 2,4-D to 

 growing stems of Araucaria excelsa reduced the length of the tracheids 

 by more than half and changed the pitting from circular to scalariform. 



Jacobs (1956) finds that the regeneration of severed xylem strands 

 and the distribution of auxin proceed together, and from this and other 

 evidence he concludes that auxin is usually the limiting factor in the 

 differentiation of xylem. The chief distinction of xylem cells is their 

 thick secondary wall. The facts that auxin is effective only in plants- 

 organisms with cellulose walls-and that the only plant group where 

 auxin seems to have little effect on growth is the fungi, which have 

 chitinous rather than cellulose walls, both suggest that auxin acts on the 

 cell wall. 



Native auxin and synthetic growth substances have been shown to be 

 effective in preventing the abscission of leaves and fruits. If a leaf 

 blade is removed but the petiole left, this will soon drop off by abscission. 

 If one of several growth substances is placed on the cut petiole stump, 

 however, abscission will not take place. Presumably when the leaf is in- 

 tact auxin is continually moving down the petiole and inhibits the 

 differentiation of an abscission layer at the base. The use of sprays of 

 various growth substances to prevent the fall of leaves or fruits under 

 certain conditions is now a common horticultural practice. What the 



