Part 6 (2/2)
Now Mr. Scott shows in several cases that the same law holds good when two heterostyled species of Primula are intercrossed, or when one is crossed with a h.o.m.ostyled species. But the results are much more complicated than with ordinary plants, as two heterostyled dimorphic species can be intercrossed in eight different ways. I will give one instance from Mr. Scott. The long-styled P.
hirsuta fertilised legitimately and illegitimately with pollen from the two forms of P. auricula, and reciprocally the long-styled P. auricula fertilised legitimately and illegitimately with pollen from the two forms of P. hirsuta, did not produce a single seed. Nor did the short-styled P. hirsuta when fertilised legitimately and illegitimately with the pollen of the two forms of P. auricula. On the other hand, the short-styled P. auricula fertilised with pollen from the long-styled P. hirsuta yielded capsules containing on an average no less than 56 seeds; and the short-styled P. auricula by pollen of the short- styled P. hirsuta yielded capsules containing on an average 42 seeds per capsule. So that out of the eight possible unions between the two forms of these two species, six were utterly barren, and two fairly fertile. We have seen also the same sort of extraordinary irregularity in the results of my twenty different crosses (Tables 2.14 to 2.18), between the two forms of the oxlip, primrose, and cowslip. Mr. Scott remarks, with respect to the results of his trials, that they are very surprising, as they show us that ”the s.e.xual forms of a species manifest in their respective powers for conjunction with those of another species, physiological peculiarities which might well ent.i.tle them, by the criterion of fertility, to specific distinction.”
Finally, although P. veris and vulgaris, when crossed legitimately, and especially when their hybrid offspring are crossed in this manner with both parent-species, were decidedly more fertile, than when crossed in an illegitimate manner, and although the legitimate cross effected by Mr. Scott between P. auricula and hirsuta was more fertile, in the ratio of 56 to 42, than the illegitimate cross, nevertheless it is very doubtful, from the extreme irregularity of the results in the various other hybrid crosses made by Mr.
Scott, whether it can be predicted that two heterostyled species are generally more fertile if crossed legitimately (i.e. when opposite forms are united) than when crossed illegitimately.
SUPPLEMENTARY NOTE ON SOME WILD HYBRID VERBASc.u.mS.
In an early part of this chapter I remarked that few other instances could be given of a hybrid spontaneously arising in such large numbers, and over so wide an extent of country, as that of the common oxlip; but perhaps the number of well-ascertained cases of naturally produced hybrid willows is equally great.
(2/19. Max Wichura 'Die b.a.s.t.a.r.dbefruchtung etc. der Weiden' 1865.) Numerous spontaneous hybrids between several species of Cistus, found near Narbonne, have been carefully described by M. Timbal-Lagrave (2/20. 'Mem. de l'Acad. des Sciences de Toulouse' 5e serie tome 5 page 28.), and many hybrids between an Aceras and Orchis have been observed by Dr. Weddell. (2/21. 'Annales des Sc.
Nat.' 3e serie Bot. tome 18 page 6.) In the genus Verbasc.u.m, hybrids are supposed to have often originated in a state of nature (2/22. See for instance the 'English Flora' by Sir J.E. Smith 1824 volume 1 page 307.); some of these undoubtedly are hybrids, and several hybrids have originated in gardens; but most of these cases require, as Gartner remarks, verification. (2/23. See Gartner 'b.a.s.t.a.r.derzeugung' 1849 page 590.) Hence the following case is worth recording, more especially as the two species in question, V. thapsus and lychnitis, are perfectly fertile when insects are excluded, showing that the stigma of each flower receives its own pollen. Moreover the flowers offer only pollen to insects, and have not been rendered attractive to them by secreting nectar.
I transplanted a young wild plant into my garden for experimental purposes, and when it flowered it plainly differed from the two species just mentioned and from a third which grows in this neighbourhood. I thought that it was a strange variety of V. thapsus. It attained the height (by measurement) of 8 feet! It was covered with a net, and ten flowers were fertilised with pollen from the same plant; later in the season, when uncovered, the flowers were freely visited by pollen-collecting bees; nevertheless, although many capsules were produced, not one contained a single seed. During the following year this same plant was left uncovered near plants of V. thapsus and lychnitis; but again it did not produce a single seed. Four flowers, however, which were repeatedly fertilised with pollen of V. lychnitis, whilst the plant was temporarily kept under a net, produced four capsules, which contained five, one, two, and two seeds; at the same time three flowers were fertilised with pollen of V. thapsus, and these produced two, two, and three seeds. To show how unproductive these seven capsules were, I may state that a fine capsule from a plant of V. thapsus growing close by contained above 700 seeds. These facts led me to search the moderately-sized field whence my plant had been removed, and I found in it many plants of V. thapsus and lychnitis as well as thirty-three plants intermediate in character between these two species. These thirty-three plants differed much from one another. In the branching of the stem they more closely resembled V.
lychnitis than V. thapsus, but in height the latter species. In the shape of their leaves they often closely approached V. lychnitis, but some had leaves extremely woolly on the upper surface and decurrent like those of V. thapsus; yet the degree of woolliness and of decurrency did not always go together. In the petals being flat and remaining open, and in the manner in which the anthers of the longer stamens were attached to the filaments, these plants all took more after V. lychnitis than V. thapsus. In the yellow colour of the corolla they all resembled the latter species. On the whole, these plants appeared to take rather more after V. lychnitis than V. thapsus. On the supposition that they were hybrids, it is not an anomalous circ.u.mstance that they should all have produced yellow flowers; for Gartner crossed white and yellow-flowered varieties of Verbasc.u.m, and the offspring thus produced never bore flowers of an intermediate tint, but either pure white or pure yellow flowers, generally of the latter colour. (2/24. 'b.a.s.t.a.r.dzeugung' page 307.)
My observations were made in the autumn; so that I was able to collect some half-matured capsules from twenty of the thirty-three intermediate plants, and likewise capsules of the pure V. lychnitis and thapsus growing in the same field. All the latter were filled with perfect but immature seeds, whilst the capsules of the twenty intermediate plants did not contain one single perfect seed. These plants, consequently, were absolutely barren. From this fact,--from the one plant which was transplanted into my garden yielding when artificially fertilised with pollen from V. lychnitis and thapsus some seeds, though extremely few in number,--from the circ.u.mstance of the two pure species growing in the same field,--and from the intermediate character of the sterile plants, there can be no doubt that they were hybrids. Judging from the position in which they were chiefly found, I am inclined to believe they were descended from V.
thapsus as the seed-bearer, and V. lychnitis as the pollen-bearer.
It is known that many species of Verbasc.u.m, when the stem is jarred or struck by a stick, cast off their flowers. (2/25. This was first observed by Correa de Serra: see Sir J.E. Smith's 'English Flora' 1824 volume 1 page 311; also 'Life of Sir J.E. Smith' volume 2 page 210. I was guided to these references by the Reverend W.A. Leighton, who observed this same phenomenon with V. virgatum.) This occurs with V. thapsus, as I have repeatedly observed. The corolla first separates from its attachment, and then the sepals spontaneously bend inwards so as to clasp the ovarium, pus.h.i.+ng off the corolla by their movement, in the course of two or three minutes. Nothing of this kind takes place with young barely expanded flowers. With Verbasc.u.m lychnitis and, as I believe, V.
phoeniceum the corolla is not cast off, however often and severely the stem may be struck. In this curious property the above-described hybrids took after V.
thapsus; for I observed, to my surprise, that when I pulled off the flower-buds round the flowers which I wished to mark with a thread, the slight jar invariably caused the corollas to fall off.
These hybrids are interesting under several points of view. First, from the number found in various parts of the same moderately-sized field. That they owed their origin to insects flying from flower to flower, whilst collecting pollen, there can be no doubt. Although insects thus rob the flowers of a most precious substance, yet they do great good; for, as I have elsewhere shown, the seedlings of V. 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 ident.i.ty 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 ill.u.s.tration 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-p.r.o.nounced exceptions to this rule occur; and here we have two species of Verbasc.u.m 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.
h.o.m.ostyled 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 f.a.gopyrum.
Various other heterostyled genera.
Rubiaceae.
Mitch.e.l.la 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.
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