The role of plant growth regulators 58 MATERIALS AND METHODS 61 Plant Material 61 Cultivar selection 61 Seed lots 61 Seedling production 62 Hypocotyl length and weight measurement 63 Light Microscopy 65 ill Specimen collection, fixation, and sectioning 65 Observation of hypocotyl anatomy 66 RESULTS 69 Hypocotyl Morphology Hypocotyl length Percent dry matter Hypocotyl swelling index Hypocotyl diameter by calculation Anatomical Measurements Cross-section diameter 89 Cortex width 92 Cortex cell number and size 95 Vascular cylinder diameter 101 Pith diameter 102 Pith cell number and cell size 108 DISCUSSION 114 SUMMARY 125 LITERATURE CITED 128 ACKNOWLEDGMENTS 151 APPENDIX 153 Analysis of variance of hypocotyl length 156 by day for Amsoy 11, Beeson 80, Corsoy 19, and Oakland soybean grown at 20, 25, or 30 C Analysis of variance for percent dry matter, 158 hypocotyl swelling index (HSI), and calculated hypocotyl diameter (HD) for Amsoy 71, Beeson 80, Corsoy 79, and Oakland soybean grown at 20, 25, and 30 c