Menyanthes trifoliata : 84. Limnanthemum Indicum : 100. Villarsia (sp.?) : 75. Forsythia suspensa : 94. Cordia (sp.?) : 100. Gilia pulchella : 100. Gilia micrantha : 81. Sethia acuminata : 83. Erythroxylum (sp.?) : 93. Cratoxylon formosum : 86. Mitchella repens, pollen-grains of the long-styled a little smaller. Borreria (sp.?) : 92. Faramea (sp.?) : 67. Suteria (sp.?) (Fritz Muller) : 75. Houstonia coerulea : 72. Oldenlandia (sp.?) : 78. Hedyotis (sp.?) : 88. Coccocypselum (sp.?) (Fritz Muller) : 100. Lipostoma (sp.?) : 80. Cinchona micrantha : 91. TRIMORPHIC SPECIES. Column 1: Name of species. Column 2: Ratio expressing the extreme differences in diameter of the pollen- grains from the two sets of anthers in the three forms. Lythrum salicaria : 60. Nesaea verticillata : 65. Oxalis Valdiviana (Hildebrand) : 71. Oxalis Regnelli : 78. Oxalis speciosa : 69. Oxalis sensitiva : 84. Pontederia (sp.?) : 55. Column 1: Name of species. Column 2: Ratio between the diameters of the pollen-grains of the two sets of anthers in the same form. Oxalis rosea, long-styled form (Hildebrand) : 83. Oxalis compressa, short-styled form : 83. Pontederia (sp.?) short-styled form : 87. Pontederia other sp. mid-styled form : 86. We here see that, with seven or eight exceptions out of the forty-three cases, the pollen-grains from one form are larger than those from the other form of the same species. The extreme difference is as 100 to 55; and we should bear in mind that in the case of spheres differing to this degree in diameter, their contents differ in the ratio of six to one. With all the species in which the grains differ in diameter, there is no exception to the rule that those from the anthers of the short-styled form, the tubes of which have to penetrate the longer pistil of the long-styled form, are larger than the grains from the other form. This curious relation led Delpino (as it formerly did me) to believe that the larger size of the grains in the short-styled flowers is connected with the greater supply of matter needed for the development of their longer tubes. (6/2. ‘Sull’ Opera, la Distribuzione dei Sessi nelle Piante’ etc 1867 page 17.) But the case of Linum, in which the grains of the two forms are of equal size, whilst the pistil of the one is about twice as long as that of the other, made me from the first feel very doubtful with respect to this view. My doubts have since been strengthened by the cases of Limnanthemum and Coccocypselum, in which the grains are of equal size in the two forms; whilst in the former genus the pistil is nearly thrice and in the latter twice as long as in the other form. In those species in which the grains are of unequal size in the two forms, there is no close relationship between the degree of their inequality and that of their pistils. Thus in Pulmonaria officinalis and in Erythroxylum the pistil in the long-styled form is about twice the length of that in the other form, whilst in