Figure 4.

NPA induces tubular leaf bases in P4 leaves. A through C, Light micrographs of representative serial, transverse sections through maize shoot apices cultured for 14 d in NPA-free media. A, Above the SAM; B, at the level of the first leaf primordium; C, below the first leaf primordium. Note that leaf primordia have separated, thin, tapered margins. Arrows show P4 (plastochron stage 4, such that a P4 leaf is the fourth leaf removed from the SAM as described in “Materials and Methods”) margins in A through C and in G. Equivalent shoots cultured in NPA show tubular leaf bases in P4 leaves (arrows in D through F and H) that result from the fusion of margin domains. I, Samples cultured for 14 d in NPA and switched to NPA-free media for 20 d have initiated three new leaf primordia (labeled 6–8 according to total leaf number as described in Fig. 2 and “Materials and Methods”) yet retain the fused, lower leaf margins of leaf 2, which was at the P4 stage of development when placed in NPA culture. The outer leaf (leaf 1) was removed to aid in fixation. Bars in A through F and in I = 100 μm. Bars in G and H= 250 μm.