OCR Text |
Show 156 D.A.RWINISl\1 CHAP. together, though both are known to be perfectly fertile with other males and fema.lcs. Cases of this kind have occurred among horses, cattle, pigs, dogs, and pigeons ; and tho experiment has been tried so frequently that there can be no doubt of the fact. Professor G. J. Romanos states that he lm: a number of a.dditional cases of this individual incompa. tibility, or of absolute sterility, between two individuals, each of which is perfectly fertile with other individuals. During the numerous experiments that have been made on the by hridisation of plants similar peculiari tics ha vo been notiecd, some individuals being capable, others inca. pablo, of being cro.~scd with a distinct species. The s:tmc individual peculitLrities are found in varieties, species, and genom. Kolreuter crossed five varieties of the common tobacco (Nicotiana. tabacum) with a distinct species, Nicotiawt glutinosa, and they n.ll yielded very sterile hybrids; buL those raised 'from one variety were less sterile, in all the experiments, than the hybrids from the four other varieties. Again, most of the species of the genus Nicotitwa have been crossed, and freely produce hybrids; but one species, N. acuminata, not particularly distinct from the others, couhl neither fertilise, nor be fertilised by, any of the eight other species experimented on. Among genera we find somesud:~ as Hippeastrum, Crinurn, Calccola.ria, Dianthus-a.lmosL all the species of which will fertilise other species and procluce hybrid offi pring; while other a.llied genera, as Zephynw tlws and Silene, notwithstanding the most persevering efforts, have not produced a single hybrid even between the most closely allied species. Dimorphism and Trimorphism. Peculiarities in the reproductive system affecting indi vidtmls of the same species reach their maximum in what are called hcterostylcd, or dimorphic and trimorphic flowers, the phenomena presented by which form one of the most remarkable of Mr. Da.rwin's many discoveries. Our eommon cowslip and primrose, as well as many other speci 'S of tho genus Primula, have two kinds of flowers in about equal proportions. In one kind the stamens are short, being situa.tcd about the middle of the tube of the corolla, while the VII ON TilE INFERTILI'£Y OF CROSSES 157 st,ylc is long, the globular stigma appearing just in the centre of tho ?PC~ Hower. In tho other kind tho stamens arc long, appca•?ng m the centro or throat of the flower, while the style 1s short, the stigma bcinrr situated halfway down tho tube at the same level as the stamens in the other form. These two forms have long been known to florists as tho " pm. -_cyc d" an d t h c "thrum-eyed," but they arc called by Darwm tho long-styled and short-styled forms (sec woodcut). Long-stylet! form. Short-sty1cd fonn. Fw. 17.-Primula voris (Cowslip). The meaning and usc of these different forms wa.s quite un_known till Darwin discovered, first, tha.t cowslips and p~·1~_roscs are absolutely barren if insects arc prevented from v1s1tmg thm_n, and then, ':hat is still more cxtraonlinary, that each form 1s ~lmost_ stcnl_e when fertilised by its own poll en, and comparatively mfert1le when crossed with any other plant of its own f~rrn, bu_t is perfectly fertile when the pollen of a long~styled 1s carn~d to the stigma of a short-styled plant, or vzce _versa. It w1ll be seen, by the figur 's, that the arrangement Is such that a bee visiting the flowers will carry the po~len from tho long a.nthers of the short-styled form to the st1gma of the long-styled form, while it would never rea.ch the stigma of another pla.ut of the short-styled form. |