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Show 322 DARWINISM CHAP. sand·wort (Spergula), and some will~w-herbs \Epilobium) .; or they arch over the pistil, as in ~a,hum a~n.rm.e ~nd Ahsma. Plantago. The style is a.lso modrfie~ to brmg 1t mto contact with the anthers, as in the dandehon, groundse.l, a.nd many other plants.l All these, however, may be occasiOnally cross-fertilised. 3. In other cases precautions arc taken to prevent cross-fertili s<ttion, as in the numerous cleistogamous or closed ftow~rs. These occur in no less than fifty-five difi'erent genera, bclongmg to twenty-four natural orders, and in thirty-two of these genera. the normal flowers are irregular, and have therefore hccn specially modified for insect fertilisation. 2 These flowers appcn.r to be degradations of the normal flowers, and arc closed u1~ hy vn.rious modifica.tions of the petals o: ot~cr parts, so that It n; impossible for insects to reach the mtcrw~·, yet they prodt~cc seed in ahunda.nce, and arc often the chiC£ means b.y whH'h the species is continued. Thus, in our co~mon dog-~wlct the perfect flowers rarely pr?duce seed, while the ru(h.men ~ar.y cleistocramic flowers do so m abundance. The sweet vwlet ,t.bo produ;cs abundtt.nce of seed from i~s cl~isto~ai?ic flowers, aml few from its perfect flowers; but m Liguna 1t produces only perfect flowers which seed abunda~tly. ~o case appears to be known of a plant which has clmstogar~uc flo,~c.rs 0~1ly, lmt a small rush (Juncus bufonius) is in th1s cond1t10n m f:\Ome parts of Russia, while in other parts perfect flow.crs arc also produced.3 Our common hcnbit dead-nettle (Lammm ampl.cxicaule) produces cleistogamic flowers, as ~o also some orchHls. The advanta.ge gained by the plant 1s great economy of specialised material, since with very small flowm~s and very little expenditure of pollen an abundance o~ seed IS pr~duce(l. 4. A considerable number of plants which have cvHlcntly been specially modified for insect fertilisation have, by further 1 The above examples are taken from Rev. G. Henslow:s paper on "Hel~·Fertlli~ ation of Plants," in T·rans. Limz. Soc. Secon<l sen es, Botwzy, Yo!: 1. pp. 317-398, with plate. Mr. H. 0. Forbef< has shown t?at the san_w th~ng occurs amona tropical orchids, in his paper "On the Contnvances f~r msunng Self-Fertilis;tion in some 'l'ropical Orchids," J ottrn. L inn. Soc., xx1. P· 538. 2 These are the numbers given by Darwin, but I am info~·me!l hy I\1r. Hemsley that many additicns have been since maue to the list, and that cleistogamic flowers probably occur in nearly all the na~u~al ~nlcr~. . a For a full account of cleistogamic flowers, sec DanVln s l'onns nf Flolne;s, chap. viii. XI THE SPECIAL COLOURS OF PLANTS 323 modification, become quite self-fertile. This is the case with the garden-pea, and also with our beautiful bee-orchis in which the polle~-masses constantly fall on to the stigmas' and the flower, bemg thus s~lf-fertilised, J?roduces abundance ~f capsules and of seed. Yet m many of Its close allies insect agency is absol~tely requir~d; but in one of these, the fly-orchis, comparatively very httle seed is produced, and self-fertilisation would t~er~fore be adva.n~ageous to it. When garden-peas were art1fic1ally cross-fertihsed by Mr. Darwin, it seemed to do t~em no good, as the seeds from these crosses produced less VIgorous plants than seed from those which were self-fertilised; a_ ~act d1rectly opposed to what usually occurs in cross-fer-tilised plants. . 5. As opposed to the theory that there is any absolute need for cross-fertilisation, it has been urged by Mr. Henslow and others that many self-.fertilised plants are exceptionally vigorous, such as groundsel, ch.ICkweed, sow-thistle, buttercups, and other common weeds; while most plants of world-wide distribution are self-fertilised, and these have proved themselves to be best fitted to survi~e. in the battle of life. More than fifty species of com~on Bntish pla~~s are very widely distributed, and all are habitually self-fertihsed.1 That self-fertilisation has some grea~ adv~ntage is shown by the fact that it is usually the spe~1es which have the smallest and least conspicuous flowers whi~h have spread widely, while the large and showy flowered species of. ~he same genera or families, which require insects to cross-fert1~1se them, have a much more limited distribution. 6 .. It IS now .believed by some botanists that many inco. nspiCu~~s and Imperfect flowers, including those that are wmd-fertihsed, such as plantains, nettles, sedges, and grasses, do not ~epresent primitive or undeveloped forms, but are degradatiOns from more perfect flowers which were once adapted to insect fertilisation. In almost every order we find so.me plants whi?~ h~ve become thus reduced or degraded for wmd or self-fertih.satwn.' as Poterium and Sanguisorba among th~ Rosac~re; while this has certainly been the case in the cle1stogam1C flowers. In most of the above-mentioned plants there are distinct rudiments of petals or other floral oraans 1 b ' Henslow's "Self-Fertilisation," Trctns. Linn. Soc. Second series, Botany, vol. i. p. 391. |