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Show 252 CONCLUDING REMARKS CHAP. VI. that the grains of the two forms should differ in size relatively to the length of the pistil which the tubes have to penetrate, but that in the latter case it would not be necessary that the grains should thns differ. Whether this explanation can be considered satisfactory must remain at present doubtful. rrhere is another relnarkable difference between the forms of several heterosty led species, na1nel y in the anthers of the short-styled flowers, which contain the larger pollen-grains, being longer than those of the long-styled flowers. This is the case with Iiottonia palustris in the ratio of 100 to 83. With Limnanthemu1n Indicum the ratio is as 100 to 70. vVith the allied Menyanthes the anthers of the short-styled form are a little and with Villarsia conspicuously larger than those of the long-sty led. With Pulmonaria angustijolia they vary much in size, but from an average of seven measurements of each kind the ratio is as 100 to 91. In six genera of the Rubiacem there is a similar difference, either slightly or well marked. Lastly, in the trimorphic Pontederia the ratio is 100 to 88 ; the anthers from the longest stamens in the short-sty led form being com pared with those from the shortest stamens in the long-sty led form. On the other hand, there is a similar and well-marked difference in the length of the stamens in the two forms of Forsythia suspensa and of Linum flavum ; but in these two cases the anthers of the short-styled flowers are shorter than those of the long-sty led. The relative size of the anthers was not particularly attended to in the two forms of the other heterosty led plants, but I believe that they are generally equal, as is certainly the case with those of the common primrose and cowslip. The pistil differs in length in the two forms of every CHAP. VI. ON HErrEROSTYLED PLANTS. 253 heterostyled plant, and although a similar difference is very general with the stamens, yet in the two forms of Linum grandijlorum and of Cordia they are equal. There can hardly be a doubt that the relative length of these organs is an adaptation for the safe tr asportal by insects of the pollen fro1n the one form to the other. The exceptional cases in which these organs do not stand exactly on a level in the two forms may probably be explained by the manner in which the flowers are visited. With most of the species, if there is any difference in the size of the stigma in the two forms, that of the long-styled, whatever its shape may be, is larger than that of the shortstyled. But here again there are some exceptions to the rule, for in the short-styled form of Leucosmia Burnettiana the stigmas are longer and much narrower than those of the long-styled; the ratio between the lengths of the stigmas in the two forms being 100 to 60. In the three Rubiaceous genera, Faramea, Houstonia and Oldenlandia, the stigmas of the short-styled form are likewise somewhat longer and narrower; and in the three forms of Oxalis sensitiva the difference is strongly marked, for if the length of the two stigmas of the long-styled pistil be taken as 100, it will be represented in the mid- and short-sty led forms by the numbers 141 and 164. As in all these cases _the stigmas of the short-style<l pistil are seated low down within a more or less tubular corolla, it is probable that they are better fitted by being long and narrow for brushing the pollen off the inserted proboscis of an insect. With many heterostyled plants the stigma differs in roughness in the two forms, and when this is the case there is no known exception to the rule that the papillm on the stigma of the long-styled form are longer |