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Show 304 GYNO-DHECIOUS PLANTS. CHAP. VII. We may now consider the probable means by which so many of the Labiatre have been separated into two forms, and the advantages thus gained. H. Muller* supposes that originally some individuals varied so as to produce more conspicuous flowers; and that insects habitually visited these first, and then dusted with their pollen visited and fertilised the less conspicuous flowers. The production of pollen by the latter plants would thus be rendered superfluous, and it would be advantageous to the species that their sta1nens should abort, so as to save useless expenditure. They would thus be converted into females. But another view may be suggested: as the production of a large supply of seeds evidently is of high i1nportance to many plants, and as we have seen in the three foregoing cases that the females produce many 1nore seeds than the hermaphrodites, increased fertility semns to 1ne the more probable cause of the formation and separation of the two forms. Fro1n the data above given it follows that ten plants of Thymus serpyllun~, if half consisted of hermaphrodites and half of fe1nales, would yield seeds compared with ten hermaphrodite plants in the ratio of 100 to 72. Under sin1ilar circumstances the ratio with Satureia hortensis (subject to the doubt from the self-fertilisation of the hermaphrodite) woHld be as 100 to 60. Whether the two forms originated in certain individuals varying and producing more seed than usual, and conseq uen tl y producing less pollen ; or in the stamens of certain individuals tending from some unknown cause to abort, and consequently producing more seed, it is impossible to decide ; but in either case, if the tendency to the increased production of seed were steadily favoured, the result would be the * 'Die Befruchtung der Blumen,' pp. Bl9, 326. CHAP. VII. GYNO-DICECIOUS PLANTS. 305 co;mplete abortion of the male organs. I . shall presently discuss the cause of the smaller s1ze of the female corolla. Scabiosa arvensis (Dipslitceoo).-It has been shown by H. Muller that this species exists in Germany under an hermaphrodite and female form.* In my neighbourhood (Kent) the female plants donotnearlyequal in number the hermaphrodites. The stamens of the females vary 1nuch in their degree of abortion; in son1e plants they are quite short and produce no pollen; in others they reach to the mouth of the corolla, but their anthers are not half the proper size, never dehisce, and contain but few pollengrains, these being colourless and of small diameter. The hermaphrodite flowers are strongly proterandous, and H. Miiller shows that, whilst all the stigmas on the same flower-head are mature at nearly the same time, the stamens dehisce one after the other; so that there is a great excess of pollen, which serves to fertilise the female plants. As the production of pollen by one set of plants is thus rendered superfluous, their male organs ha-ve become more or less completely aborted. Should it be hereafter proved that the female plants yield, as is probable, more seeds than the hermaphrodites, I should be inclined to extend the same view to this plant as to the Labiatoo. I have also observed the existence of two forms in our endmnic S. succisa, and in the exotic S. atro-purpurea. In the latter plant, differently to what occurs in S. arvensis, the female flowers. especially the larger circumferential ones, are smaller than those of the hermaphrodite form. According · to Lecoq, the female flower-heads of S. succisa are likewise smaller than those of what he calls the male plants, but which are · probably herrna phrodi tes. Echium vulgnre (Boragineoo).-The ordinary hennaphrodite form appears to be proterandrous, and nothing more need be said about it. The female differs in having a much smaller corolla and shorter pistil, but a well-developed stigma. The stamens * 'Befruchtung der Blum en ' &c., p. 368. The ... two forms occu~· not only in Germany, but in England and France. Lecoq (' Geographie But.' 1857, tom. vi: pp. 473, 477) says that male plants as well as hermaphrodites and females co-exist; it is, however, possible that he may have been deceived by the flowers being so ~trongly proterandrous. From what Lecoq says, S. succisa likewise appem·s to occur under two forms in France. X |