FORMATION OF VASCULAR PATTERN IN DEVELOPING LEAVES OF THE C-4 GRASS ARUNDINELLA-HIRTA

In leaves of typical C-4 species, specialized bundle sheath and mesoph yll cells are positioned in relation to the veins, and the precursors of leaf vasculature form a template for the spatial pattern of photosy nthetic cell differentiation. In this study we used reconstructions fr om serial sections and clearings to describe the formation of vascular pattern formation in developing leaves of the C-4 grass Arundinella h irta with the long-term goal of understanding the role that vascular p attern formation might play in later stages of cellular differentiatio n. Arundinella hirta is unusual in that an additional photosynthetic c ell type, bundle sheath-like ''distinctive cells,'' form longitudinal strands within the mesophyll. We also describe the pattern of distinct ive cell file formation. During leaf morphogenesis and early extension growth, procambial strands of longitudinal veins appear sequentially and are interpolated between earlier-formed procambial strands at regu lar intervals. Distinctive cell files arise after the procambial stran ds of smallest veins and follow a similar pattern, with the exceptions that distinctive cell files form simultaneously and at closer interva ls than do vein procambial strands. Distinctive cell files and vein pr ocambium (with the exception of the median vein) develop in isolation from the stem vascular system, a pattern indicating that interpretatio n of positional information by competent cells might be the mechanism of vascular pattern formation. Mature spatial patterns and the pattern of formation support the homology of distinctive cell files with smal l longitudinal veins but indicate that shared developmental pathways h ave been modified.