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Show 308 DARWIN! M CHAP. than as regards conspicuousness, hence n. tendency to ~~ny decided colour hn.s been preserved and acc~mulated as ~crvmg to render the fruit easily visible amon~ 1ts s~rroun~lmfs ?f leaves or herbage. Out of 134 frmt- bc~rmg .1? an · m Mon()'redicn's 1~rees and Shrubs, and Hookers B1·1t~sh Flon~, the f~uits of no less than sixty-eight, or rather more tha,n h~lf, arc red, forty-five arc black, four~ce~ yellow, and s?vcn white. The great prevalence of red frmt~ 1s almost certa~nly. due to their greater conspicuousness .hav1~g favoured thmr <h. ·pcr: al, thoucrh it may also have ansen m part from the chmm.c<d chan~cs of chlorophyll during ripening and decay prodnCl.ng rod tints ·as in many fa-ding leaves. Yet the comparat1_ve scarcity of yellow in fruits, w.hilc it is. t?e most common tmt of fa,ding leaves, is agn.inst th.ls suppos1t10n. . . There axe, however, a few mstanccs of coloured frUits which do not seem to he intended to be eaten; such arc th ' c·olucynth plant (Cucumis colocynthus), which has a bm1Utifnl fn_!iL the size and colour of an orange, but na,uscous beyond <l escnption to the taste. It hn,s a h<trd rind, a,nd may perhapi-i lw di spersed by being blown a.loi!g. the wound,_ the colm~rhei ng .:\ tl a-dventitious product; but It 1s qmto po.·s1hlc, notw1th. t:t~Hhng its repulsivcn ss to us, thttt it may be C<ttcn by some ~U l l mal > \Vith rccrard to the fruit of another plant, Calotropu; proccra, there is le.ss doub~, as it is dry an.cl fnll uf thin, flat-winged seeds, w1th :fi1~e ~1lky :fibmc~1ts, enuncntly :Hlaptcd for wind-dispersal; yet 1t IS of a bnght ye~low colum, :Ii-i ];trge as an apple, and therefore very c_onsp!cuou.·. H<~rc, therefore we seem to have colour wh1ch IS ~t m 're liyproduct 'of tho organism and of .no u~c to it; but i-illch cases arc exceedingly rare, and th1s ra.nty, when compared with the great abundance of cases in. which th ere _ii-i an ol vious purpose in the colour, add wmght to ~ho cvulc_nce in favour of the theory of the attmctive coloratwu of <:<hh~ · fruits in order that birds a,nd other anim::.tl. may a.·s1:-;t 111 their dispersal. Both the ::tbovo-namcd plants arc natives of Palestine and the adjacent arid coun trio ·.1 The Colours of Flowers. Flowers are much more varied in their colours thnn fruits, 1 Canon Tristram's Nat11/i'alllistory of the Bible, pp. 4 3, 4 .J.. XI THE SPECIAL OLOURS OF PLANTS 309 as they arc more complex and more varied in form and structure; yet there is some parallelism between them in both respcc_ts. Flowers aec frequently adapted to attract insects as frmts are to a~~rac~ bir~s, the object being in the former to secure cr~ s-fcr~Ihsatwn, m the latter dispersal; while jnst as colon: 1~ an m _lex of the edibility of fruits which supply pulp or JUICe to bu·ds, so arc the colours of flowers an indication of the presence of nectar or of pollen which are devoured by insects. The ma.in facts and many of tho details, as to the relation of insc~ts to flower~, were discovered by Sprongel in 1793. He notJCed the cu:·wus ~claptation ?f t~e structure of many flowe:· to the particular. ~nscct whiCh v1sit them; he proved that 1nsccts do cross-fert1hso flowers, and he believed tha,t this was the object of the adaptation •·, while the presence of nectar a~d poll~n en. ~tred the contim~~tnco of their visits; yet he m1s cd d1scovormg the use of th1s cross-fertilisation. Several writers at a later period obtained evidence that cross-fertilisation ~f plants was. a ?e~cfit to them; but the wide generality of th1s fact and Its mt1matc connection with the numerous and curious adaptations discovered by Sprongel, was :first shown by Mr. Darwi1~, and has since been demonstrated by a vast ma.ss of observatwns, foremost among which arc his own researches on orchids, primula , and other plants.l By an clabora~e series of experiments carried on for many years Mr. Darwm demonstrated the great value of crossfertilisation in increasing tho rapidity of o-rowth tho strength d . f b ' an vigour o the plant, and in ad<ling to its fertility. This ·effect is produced immediately, not a,s he expected would be the case, after several generations of crosses. He planted seeds from cross-fertilised and self-fcrtili cd pl::tnts on two ides of the same pot exposed to exactly simnar conditions, and in mo~t ca c. the difference in size n.nd vigour was a,mazing, wh1le the plants from cross-fertilised parents also produced more and .finer seeds. These experiments entirely confirmed the cxponcncc of breeders of animal already referred to {p. 160), and led him to enunciate his famous aphorism, 1 For a complete. hist~t'ical account of this subject with full references •to all the works upon tt, see the Introduction to Hermann Muller's Fertilisation cj Flower·s, trauslo.tcu by D'Arcy W. Thompson. |
