Figure 7.15 Leaf initiation and shape is determined by auxin and KNOX genes. (a) Diagram of the shoot apex with central meristem, incipient leaves I1-I3, and initiated leaves P1-P5. Arrows indicate the flow of auxin to the site where a new leaf will initiate. (b) in situ hybridisation showing expression of the class I KNOX gene SHOOT MERISTEMLESS in the shoot meristem and down-regulation in initiating and developing leaves. (c) Young Arabidopsis wild type seedling showing two cotyledons and first leaves. (d) The class I KNOX gene mutant shoot meristemless (stm) has 2 cotyledons and fails to produce leaves due to lack of a shoot meristem.