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Show I I I ~ 412 SEXUAL RELATIONS OF PLAN'l'S. CnAP. X. holds good generally, and it certainly does not do so in Australia. But I have been assured that tho flowers of the prevailing Australian trees, na1noly, the Myrtacec.e, swarm with insects, and if they arc dichogainous they would be practically diclinous. ·~ .As :far as ~nemophilous plants are concerned, we know thatthey are apt to have their sexes separated, ancl we can see that it would be an unfavourable circumstance for them to bear their flowers very close to tho ground, as their pollen is liable to be blown high up in tho air; t but as the culms of grasses give sufficient elevation, we cannot thus account for so many trees ancl bushes being diclinous. We rna y infer from our previous discussion that a tree bearing numerous hermaphrodite flowers would rarely intercross with another tree, except by means of the pollen of a distinct individual being prepotent over the plant's own pollen. Now the separation of the sexes, whether the plant were anomophilous or entomophilous, would most effectually bar self-fertilisation, and this may be the cause of so many trees and bushes being diclinous. Or to put the case in another way, a plant would be bettor :fitted for development into a tree, if the sexes were separated, than if it were hermaphrodite; for in tho fonncr case its numerous flowers would be less liable to continued Asa Gray informs me that in the United States there are 132 native trees (belonging to twenty-£. ve fami~ies) of which ninety-five (belongmg to seventeen families) "have their sexes more or less separated, for the greater part decidedly separated." * With respect to the Proteacere of Australia, Mr. Bentham remarks (' Journal Linn. Soc. Bot.' vol. xiii. 1871, pp. 58, 64:) on the various contrivances by which the stigma in the sovorn1 genera is screened from tho actiou of the pollen from tile . nmc fiowcr. . For instanc , in Synaphon, ''tho st1gma it:l hold. by the eunuch (i.e., one of tlto stamens which is barren) safe from all pollution from her brother nnthers, and i:; preserved intact for any pollen that may be insorte'l by insects and ot!Jer agencies." . t Kerner, ' SchutzmLttel des Pollens,' 1873, p. 4. CHAP. X. SEXUAL HELATIONS OF PLANTS. 413 self-fertilis. ation. But it should also b e ob serve d that the long hfe of a tree or bush I)ermits of th . . f . th · o separation o e sexes, with much less risk of evil f· · t . . Iom Impreg-na Ion occasionally failincr and seeds not b · d d h . o eing pro-nee , t an In the case of short-lived plants H · b bl · · ence It pro a y Is, as Lecoq has remarked, that annual plants are rarely dimcious. .. Finally, we have seen reason to believe that the hi~her pla~ts are descended from extremely low forms w.hich c~nJngated, and that the conjugating individuals d~ffered soinewhat fron1 one another,-the one representing the male and the other the female-so that plan.ts were aboriginally dirocious. At a very early penod su~h lowly organised dimcious plants p~obably gave nse by budding to monrocious plants With the two sexes borne by the same individual · and by a sti1l ?loser union of the sexes to hermapbi~odite plants, which are now much the commonest form.* As soon as plants became affixed to the ground, their pollen must have been carried by some means from flower to flower, at first almost certainly by the wind then. by _pollen-devouring, and afterwards by nectar~ seeking 1~sects. During subsequent ages some few en~omophilous plants have been again rendered anemophilous, and some hennaphrodite plants have had their sexes again separated ; and we can vaguely see the advantages of sueh recurrent changes under certain conditions. Dioocious plants, however fertilised, have a great * There is a considerable a~ount of evidence tl1at all the higher animals arc the descendants ~f hermaphrodites; and it is a cunous problem whether such ~ermaphroditism may not have een tlle result of the conjugation of two slightly different indivi-duals, which represented the two incipient sexes. On this view, the higher animals may now owe their bilateral structure, with all their organs double at an early embryonic period, to the fusion or con,iugation of two primordial individuals. |