42 SUBCELLULAR PARTICLES 



into an intermediate of increased oxygen content, possibly corresponding to a 

 monomer radical. Thus eugenol and the other precursors recognized could be 

 more correctly termed 'premonomers.' 



Although the present paper was not intended as a treatise on the physiology 

 and biochemistry of lignification, its proper subject matter grew out of a physio- 

 logical problem and cannot attain its full significance divorced from considera- 

 tions at the cellular and organismal level. It is therefore of value to consider 

 briefly some of the chemical factors in the cell that might contribute to the 

 regulation of lignin deposition. 



Among active substances, 3-indoleacetic acid, the plant elongation hormone, is 

 of uncommon interest. This substance has been reported to promote regeneration 

 of xylem, the most highly lignified of plant tissues (16), and it is further known 

 to induce the formation of peroxidase in vascular tissue (11, 17). In contrast, 

 it has been reported (29) and confirmed (39) that indoleacetic acid can inhibit 

 lignification when supplied at concentrations somewhat higher than those in 

 the stimulatory range. In addition to the growth hormone, reducing compounds 

 such as cysteine and ascorbic acid can inhibit the conversion of eugenol to lignin 

 (29), as can common components of the metabolically active young cell. Among 

 the ordinary metabolites, the following produce sizeable inhibitions when sup- 

 plied in concentrations equimolar with eugenol: a) aspartic acid, 9o'/(, ; b) 

 arginine, lysine, glycine, 35-40%; c) cadaverine, 60%; d) glycerol, 60%; e) 

 Krebs cycle acids, 50-90%; /) adenylic acid, 90%.; g) glucose-i-phosphate, 50%; 

 and h) Mg(II), Ca(II), 50%,. Several esters and terpenes were without activity. 

 In contrast, the inhibitions produced by indoleacetic acid, from 25 to 100% 

 depending upon the tissue, were effected with only i molecule for every 20 

 of eugenol. Thus, regulation of lignin synthesis can be linked to normal hor- 

 monal and metabolic components of the plant cell. Although these factors re- 

 main to be evaluated as to comparative significance, the dual role of indoleactic 

 acid, together with the lower effective levels needed, makes it most prominent 

 as a hormonal regulator of lignification. Indoleacetic acid is particularly concerned 

 with elongation of the young post-mitotic cell and is a factor of importance in 

 the etiology of plant neoplasms such as crown gall (20). It is a hormone of 

 importance in maintenance of a relatively juvenile, although postembryonic, 

 physiological state, and its decline is particularly associated with cellular differen- 

 tiation and senescence (10). Its role in the promotion of lignification may begin 

 with the already established ability of indoleacetic acid to induce peroxidase 

 synthesis, especially in vascular tissue. In spite of increased enzyme, oxidation of 

 eugenol and other substrates is not immediately accelerated, as indoleacetic acid 

 is also a competitive inhibitor with these substrates for peroxidases (28), and is 

 itself slowly destroyed by the enzyme. Hence, as the level of induced peroxidase 

 rises, the concentration of indoleacetic acid declines and the peroxidation of other 



