thapsus raised from flowers fertilised with pollen from another plant, are more vigorous than those raised from self-fertilised flowers. (2/26. 'The Effects of Cross and Self-fertilisation' 1876 page 89.) But in this particular instance the insects did great harm, as they led to the production of utterly barren plants. Secondly, these hybrids are remarkable from differing much from one another in many of their characters; for hybrids of the first generation, if raised from uncultivated plants, are generally uniform in character. That these hybrids belonged to the first generation we may safely conclude, from the absolute sterility of all those observed by me in a state of nature and of the one plant in my garden, excepting when artificially and repeatedly fertilised with pure pollen, and then the number of seeds produced was extremely small. As these hybrids varied so much, an almost perfectly graduated series of forms, connecting together the two widely distinct parent-species, could easily have been selected. This case, like that of the common oxlip, shows that botanists ought to be cautious in inferring the specific identity of two forms from the presence of intermediate gradations; nor would it be easy in the many cases in which hybrids are moderately fertile to detect a slight degree of sterility in such plants growing in a state of nature and liable to be fertilised by either parent-species. Thirdly and lastly, these hybrids offer an excellent illustration of a statement made by that admirable observer Gartner, namely, that although plants which can be crossed with ease generally produce fairly fertile offspring, yet well-pronounced exceptions to this rule occur; and here we have two species of Verbascum which evidently cross with the greatest ease, but produce hybrids which are excessively sterile.

CHAPTER III. HETEROSTYLED DIMORPHIC PLANTS--continued.

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 officinalis, singular difference in self-fertility between the English and German long-styled plants. Pulmonaria angustifolia shown to be a distinct species, long-styled form completely self-sterile. Polygonum fagopyrum. Various other heterostyled genera. Rubiaceae. Mitchella repens, fertility of the flowers in pairs. Houstonia. Faramea, remarkable difference in the pollen-grains of the two forms; torsion of the stamens in the short-styled form alone; development not as yet perfect. The heterostyled structure in the several Rubiaceous genera not due to descent in common.

(FIGURE 3.4. Linum grandiflorum. Left: Long-styled form. Right: Short-styled form. s, s: stigmas.)

It has long been known that several species of Linum present two forms (3/1. Treviranus has shown that this is the case in his review of my original paper 'Botanische Zeitung' 1863 page 189.), and having observed this fact in L. flavum 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 (Figure 3.4). 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- styled. A more important distinction is, that the five stigmas in the short- styled form diverge greatly from one another, and pass out between the filaments of the stamens, and thus lie within the tube of the corolla. In the long-styled form the elongated stigmas stand nearly upright, and alternate with the anthers. In this latter form the length of the stigmas varies considerably, their upper extremities projecting even a little above the anthers, or reaching up only to about their middle.

The Different Forms of Flowers on Plants of the Same Species Page 36

19th Century English Literature

Charles Darwin

Free Books in the public domain from the Classic Literature Library ©

Charles Darwin

All Pages of This Book