The goal of this study was to investigate the roles of methyl radicals and acetylene, either individually or together, during diamond growth by chemical vapor deposition. We have nucleated and grown micron-sized diamond particles at 800 degreesC in a flow-tube apparatus that permits growth from only methyl radicals or acetylene in atomic hydrogen, in contrast to the complex mixture of species found in a normal reactor. Growth from methyl radicals only produced cubo-octahedral crystals with an alpha value (root3 x the ratio of growth rates in the [1 0 0] and [1 1 1] directions) near 1.8, indicating that the absence of acetylene is not a significant impediment in nucleating new (I 1 1) planes, in contradiction to recent modeling work. Diamond growth from pure acetylene produced octahedra (alpha = 3), indicating that (1 0 0) growth is much more facile than (1 1 1) growth in the absence of methyl radicals, and the (1 1 1) facets had a high concentration of contact twins. Diamond growth from acetylene plus methane produced cubo-octahedra crystals but the highly defective [1 1 1] growth persisted. We propose that at steady state the (1 1 1) growth surface has a high coverage of adsorbed hydrocarbons, rather than resembling the simple hydrogen-terminated (1 1 1)-1 x 1 : H structure; that steric repulsion and rearrangements play a critical role in the growth mechanism; and that desorption and etching of adsorbed hydrocarbons by atomic hydrogen is less facile than has been commonly supposed. The ratio of C-2 and C-1 gas-phase precursors should be minimized for high quality [1 1 1] epitaxy.