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Show 41.0 SEXUAL RELATIONS OF PLANTS, CHAP. X. disconnected. If this view is ·correct, plants must have been rendered hermaphrodites at a later though still very early period, and ento1nophilous at a yet later period, namely, after the development of winged insects. So that the relationship between hermaphroditism. and fertilisation by moans of insects is likewise to a certain extent intelligible. 'Vhy the descendants of plants which wore originally direcious, and which therefor profited by always inter .. crossing with another individual, should have been converted into hermaphrodites, may perhaps be explained by the risk which they ran, cRpecially as long as they were anemophilous, of not being always fertilised, and consequently of not leaving offspring. rr his latter evil, the greatest of all to any organis1n, would have been much lessened by their becoming hermaphrodites, though with the contingent disadvantage of frequent self-fertilisation. By what graduated steps an hcnnaphrodite condition was acquired \VC do not know. But we can see that if a lowly organised fonn, in which the two sexes were r present d by soincwhat different individuals, were to increase by budding either before or after conjugation, the two incipient sexes would be capable of appearing by buds on the sa1ne stock, as occasionally occurs with various characters at the present day. The organism would then be in a monrecious condition, and this is pro ha bl y the first step towards hermaphroditism; for if very simple male and female flowers on the saine stock, each consisting of a single stamen or pistil, wore brought do~e together and surrounded by a· common envelope, 1n nearly the same manner as with the florets of the Compositm, we should have n:n hcnnaphrodite flowe~. There seems to be no liinit to the changes whiCh organisms undergo under· changing qonditions of life; CHAP. X. SEXUAL RELATIONS OF PLANTS. 411 an~ some hermaph.ro.dite plants, descended as we must bel~eve from a~onginally diclinous plants, have hacl then sexes again separated. That this bas occurred ~ve n1ay infer from the presence of rudimentary stamen~ 1~ t~e ~owers of some individuals, and of rudimentary p1stils In ~he ~?wers of other indivi(luals, for example 1n Lychntts dttottca. But a conversion of this kind will not have occurr~d unless cross-fertilisation was already assured, gen~rally by the agency of insects; but why the production of 1nale and female flowers on distinct plants should have been advantageous to the species, cross-fertilisation having been previously assured, is far from obvious. A plant might indeed produce twice as many seeds as were necessary to keep up its numbers under new or changed conditions of life ; and if it did not vary by bearing fewer flowers, and did vary in the state of its rep rod ucti ve organs (as often occurs under cultivation), a wasteful expenditure of seeds and pollen would be saved by the flowers becoming diclinous. A related point is worth notice. I remarked in my Origin of Species that in Britain a much larger proportion of trees and bushes than of herbaceous plants have their sexes separated; and so it is, according to Asa Gray and Hooker, in North America and New Zealand.* It is, however, doubtful how far this rule . * I find in the 'London Catalogue o~Bl'itish Plants,' that there are thuty-two indigenous trees and bushes in G!'eat Bl'itnin classed unde1· nine families ; but to erl' on the safe side, I have counted only six species of willows. Of the thirty-two trees and bushes, nineteen, or more than half, have their sexes separated· a!ld this is an enol'mous pl'opor~ tion compared with other Bl'itish Pl.ants. New Zealand abounds Wlth diclinous plants and tl'ces; anrl Dl'. Hooke!' calculates that out of about 75G pbanerognmic plants inh abiting the idands, no less than 108 are trees, belonging to thirty-five families. Of these 108 tl'ces, fifty-two, or very nearly half, have their sexes more or less separated. Of bushes th01·e are 149, of which sixtyone have their sexes in the same state; whilst of the remaining 500 herbaceous pbnts only 121, Ol' less than a fourth, havu their sexe8 separated. Lastly, Prof. |
