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Show 80 HYBRID VERBASCUMS. CHAP. II. cauti• ous I• n I• n11! ern• ng the specific .i dentity of .t wo .f onns f.r o1n t h e presence of intermediate gr.a dations · ,d nor wou ld I. t b e easy I. n the many cases in whichf h ybn. s are 1 . · n1o d eratel y 11!e r t.Il e t o detect a slight degree o s. ten Ity In sueh p l an t s growi· n g I·n a state of nature. and hable to be fertilised by either parent-specie.s. Thu~ly and lastly, these hybrids offer an excellent Illustrat~on of a stateInent made by that admirable observer Gartner, _namely, that although plants which c~n be cro~sed with ease generally produce fairly f~rtile offspn~g, yet ~ellpronounced exceptions to th1s rule o.ccur , . and here. we have two species of V erbascum wh1ch e:Ident~y cross with the greatest ease, but produce hybnds wh1ch are excessively sterile. CHAP. III. LINUM GRANDIFLORUM. 81 CHAPTER III. HETEROSTYLED DIMORPHIC PLANTS-eontinued. Linum grandiflorum, long-styled form utterly sterile with own-form pollen-Linum perenne, torsion of the pistils in the long-styled form alone-Homostyled species of Linum-Pulmonaria officiualis, singular difference in self-fertility between the English and German long-styled plants-Pulmonaria angu:;tifolia shown to be a distinct species, long-styled form completely self-sterile-Polygonum fagopyrum- Various other heterostyled genera-Rubiacero-Mitchella repens, fertility of the flowers in pairs-Houstonia-Faramea, remarkable difft ·rence in the pollen-grains of the two forms; torsion of the stamens in the short-styled form alone; development not as yet perfect-The beterostyled structure in the several Rubiaceous genera not due to descent in common. IT has long been known* that several species of Linum present two forms, and having observed this fact in L. jlavum more than thirty years ago, I was led, after ascertaining the nature of heterostylism in Primula, to examine the first species of Linum which I met with, namely, the beautiful L. grandiflorum. This plant exists under two forms, occurring in about equal numbers, which differ little in structure, but greatly in function. The foliage, corolla, stamens, and pollen-grains (the latter examined both distended with water and dry) are alike in the two forms (Fig. 4) The difference is confined to the pistil; in the short styled form the styles and the stigmas are only about half the length of those in the long-sty led. A more * Treviranus l1as sh()wn that this is the case in hil; review of my original paper, 'Bot. Zeitung,' 1863, p. 189. G |
