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Show 282 DICECIOUS AND CHAP. VII. species, which exist as males, females and hermaphrodites, the latter would have to be supplanted before the species could become strictly dicecious ; but the extinction of the hermaphrodite form would probably not be difficult, as a complete separation of the sexes appears often to be in some way beneficial. The males and females would also have to be equalised in number, or produced in some fitting proportion for the effectual fertilisation of the females. There are, no doubt, many unknown laws which govern the suppression of the male or female organs in hermaphrodite plants, quite independently of any tendency in them to become moncecious, dicecious, or polygamous. We see this in those hermaphrodites which from the rudiments still present manifestly once possessed more stamens or pistils than they now do,-even twice as many, as a whole verticil has · often been suppressed. Robert Brown remarks* that "the order of reduction or abortion of the stamina in any natural family may with some confidence be predicted," by observing in other members of the family, in which their number is complete, the ·order of the dehiscence of the anthers; for the lesser permanence of an organ is generally connected with its lesser perfection, and he judges of perfection by priority of development. He also states that whenever there is a separation of the sexes in an hermaphrodite plant, which bears flowers on a simple spike, it is the females which expand first; and this he likewise attributes to the female sex being the more perfect of the two, but why the female should be thus valued he does not explain. * 'Trans. L1'nn . S oc. ' vo I• xn.. . p. 98 . Or' M1'scellaneous Works,' vol. ii. pp. 278-81. CHAP. VII. POLYGAMOUS PLANTS. 283 Plants under cultivation or changed conditions of life frequently become sterile; and the male organs are much oftener affected than the female, though the latter alone are sometimes affected. The sterility of the stamens is generally accompanied by a reduction in their size ; and we may feel sure, from a wide-spread analogy, that both the male and female organs would become rudimentary in the course of many generations if they failed altogether to perform their proper functions. According to Gartner,* if the anthers on a plant are con tabescent (and when this occurs it is · always at a very early period of growth) the female organs are sometimes precocious! y developed. I mention this case as it appears to be one of compensation. So again is the well-known fact, that plants which increase largely by stolons or other such means are often utterly barren, with a large proportion of their pollen -grains in a worthless condition. Hildebrand has shown that with hermaphrodite plants which are strongly proterandrous, the stamens in the flowers which open first sometimes abort ; and this seems to follow from their being useless, as no pistils are then ready to be fertilised. Conversely the pistils in the flowers which open last sometimes abort; as when they are ready for fertilisation all the pollen has been shed. He further shows by means of a series of gradations amongst the Compositoo, t that a. tendency from the causes just specified to produce mther male or female florets, sometimes spreads to all the florets on the same head, and sometimes * 'Beitrage zur Kenntniss,' &c. p. 117 et seq. Thew hole subj cct of !he sterility of plants from yarlOus causes has been discussed m my ' Variation of Animals and Plants under Domestication,' chap. xviii.-2nd edit. vol. ii. pp. 146-56. t 'Ueber die Geschlechtsverhaltnisse bei den Compositen,' 1869, p. 89. |
