In a study of guayule (Parthenium argentatum) in which 400 ovules of 72-chramosome type and 75 ovules of 36-chromosome type were compared, many resting mega-spore mother cells and enlarged ones (comparable to uni-nucleate embryo sacs) were found in the former, whereas in the latter many megaspore mother cells were in different stages of meiosis. From the distribution of types it is estimated that in the 72-chromosome material approx. 74.2% of apospory occurs while in the 36-chromosome plants the frequency is < 4%. The achenes from the non-pollinated flowers of a 36-chromosome plant contained no embryos; open pollinated plants of the same type contained embryos. Non-pollinated 72-chromosome plants contained embryos in most ovules, but lack of pollination reduced the % in which endosperm was developing and either reduced or inhibited the development of full-size embryos. In conjunction with generative apospory unreduced pseudogamy evidently occurs in the apomictic types.