PART I Transections of the stem of Triticum were examined after staining with dyes specific for functional groups within the lignin polymer. Anatomical observations suggest that the basis for the rapid increase in the lignin content of this plant 35 to 40 days after germination, is the differentiation of subepidermal sclerenchyma fibres in the stem at this time. The lignin formed in the fibre walls appears to have a higher methoxyl content than the lignin of the xylem vessels. A comparison of the development of lignification with stem elongation and flowering was made and the interrelationship of these processes discussed. PART II The role of p-hydroxyphenyllactic acid in lignification in wheat was investigated. ¹⁴C-labelled tyrosine, p-hydroxyphenyllactic acid (HPLA), and ³H-labelled HPLA were administered separately to the cut ends of shoots of Triticum and the incorporation of label into ethanol-soluble and ethanol-insoluble ferulic (and in some cases only, p-hydroxycinnamic) acid was measured. On the basis of the pattern of incorporation of label from the ¹⁴C-tyrosine, experiments were carried out to determine the route by which HPLA is converted to lignin precursors. A failure to detect label from ³H-HPLA in the cinnamic acids suggests that HPLA is not dehydrated directly to p-hydroxycinnamic acid and is not of regulatory significance in lignification in either 10 or 40 day-old wheat plants. PART III Information from several levels of organization within the plant is drawn together and discussed. Suggestions for further work investigating the controlling factors in lignification are included.