The most remarkable point in this table is the small average number of seeds from the short-styled flowers when illegitimately fertilised, and the unusually large average number of seeds yielded by the illegitimately fertilised long- styled flowers, relatively in both cases to the product of the legitimately fertilised flowers. (1/18. H. Muller says ‘Die Befruchtung’ etc. page 352, that the long-styled flowers, when illegitimately fertilised, yield as many seeds as when legitimately fertilised; but by adding up the number of seeds from all the capsules produced by the two methods of fertilisation, as given by him, I arrive at the results shown in Table 1.13. The average number in the long-styled capsules, when legitimately fertilised, is 91.4, and when illegitimately fertilised, 77.5; or as 100 to 85. H. Muller agrees with me that this is the proper manner of viewing the case.) The two legitimate unions compared with the two illegitimate together yield seeds in the ratio of 100 to 61.
H. Muller also tried the effects of illegitimately fertilising the long-styled and short-styled flowers with their own pollen, instead of with that from another plant of the same form; and the results are very striking. For the capsules from the long-styled flowers thus treated contained, on an average, only 15.7 seeds instead of 77.5; and those from the short-styled 6.5, instead of 18.7 seeds per capsule. The number 6.5 agrees closely with Mr. Scott’s result from the same form similarly fertilised.
From some observations by Dr. Torrey, Hottonia inflata, an inhabitant of the United States, does not appear to be heterostyled, but is remarkable from producing cleistogamic flowers, as will be seen in the last chapter of this volume.
Besides the genera Primula and Hottonia, Androsace (vel Gregoria, vel Aretia) vitalliana is heterostyled. Mr. Scott fertilised with their own pollen 21 flowers on three short-styled plants in the Edinburgh Botanic Gardens, and not one yielded a single seed; but eight of them which were fertilised with pollen from one of the other plants of the same form, set two empty capsules. (1/19. See also Treviranus in ‘Botanische Zeitung’ 1863 page 6 on this plant being dimorphic.) He was able to examine only dried specimens of the long-styled forms. But the evidence seems sufficient to leave hardly a doubt that Androsace is heterostyled. Fritz Muller sent me from South Brazil dried flowers of a Statice which he believed to be heterostyled. In the one form the pistil was considerably longer and the stamens slightly shorter than the corresponding organs in the other form. But as in the shorter-styled form the stigmas reached up to the anthers of the same flower, and as I could not detect in the dried specimens of the two forms any difference in their stigmas, or in the size of their pollen-grains, I dare not rank this plant as heterostyled. From statements made by Vaucher I was led to think that Soldanella alpina was heterostyled, but it is impossible that Kerner, who has closely studied this plant, could have overlooked the fact. So again from other statements it appeared probable that Pyrola might be heterostyled, but H. Muller examined for me two species in North Germany, and found this not to be the case.
CHAPTER II. HYBRID PRIMULAS.
The oxlip a hybrid naturally produced between Primula veris and vulgaris.
The differences in structure and function between the two parent-species.
Effects of crossing long-styled and short-styled oxlips with one another and
with the two forms of both parent-species.
Character of the offspring from oxlips artificially self-fertilised and cross-
fertilised in a state of nature.
Primula elatior shown to be a distinct species.
Hybrids between other heterostyled species of Primula.
Supplementary note on spontaneously produced hybrids in the genus Verbascum.
The various species of Primula have produced in a state of nature throughout Europe an extraordinary number of hybrid forms. For instance, Professor Kerner has found no less than twenty-five such forms in the Alps. (2/1. “Die Primulaceen-Bastarten” ‘Oesterr. Botanische Zeitschrift’ Jahr 1875 Numbers 3, 4 and 5. See also Godron on hybrid Primulas in ‘Bull. Soc. Bot. de France’ tome 10 1853 page 178. Also in ‘Revue des Sciences Nat.’ 1875 page 331.) The frequent occurrence of hybrids in this genus no doubt has been favoured by most of the species being heterostyled, and consequently requiring cross-fertilisation by insects; yet in some other genera, species which are not heterostyled and which in some respects appear not well adapted for hybrid-fertilisation, have likewise been largely hybridised. In certain districts of England, the common oxlip—a hybrid between the cowslip (P. veris, vel officinalis) and the primrose (P. vulgaris, vel acaulis)—is frequently found, and it occurs occasionally almost everywhere. Owing to the frequency of this intermediate hybrid form, and to the existence of the Bardfield oxlip (P. elatior), which resembles to a certain extent the common oxlip, the claim of the three forms to rank as distinct species has been discussed oftener and at greater length than that of almost any other plant. Linnaeus considered P. veris, vulgaris and elatior to be varieties of the same species, as do some distinguished botanists at the present day; whilst others who have carefully studied these plants do not doubt that they are distinct species. The following observations prove, I think, that the latter view is correct; and they further show that the common oxlip is a hybrid between P. veris and vulgaris.
The cowslip differs so conspicuously in general appearance from the primrose, that nothing need here be said with respect to their external characters. (2/2. The Reverend W.A. Leighton has pointed out certain differences in the form of the capsules and seed in ‘Annals and Magazine of Natural History’ 2nd series volume 2 1848 page 164.) But some less obvious differences deserve notice. As both species are heterostyled, their complete fertilisation depends on insects. The cowslip is habitually visited during the day by the larger humble-bees (namely Bombus muscorum and hortorum), and at night by moths, as I have seen in the case of Cucullia. The primrose is never visited (and I speak after many years’ observation) by the larger humble-bees, and only rarely by the smaller kinds; hence its fertilisation must depend almost exclusively on moths. There is nothing in the structure of the flowers of the two plants which can determine the visits of such widely different insects. But they emit a different odour, and perhaps their nectar may have a different taste. Both the long-styled and short-styled forms of the primrose, when legitimately and naturally fertilised, yield on an average many more seeds per capsule than the cowslip, namely, in the proportion of 100 to 55. When illegitimately fertilised they are likewise more fertile than the two forms of the cowslip, as shown by the larger proportion of their flowers which set capsules, and by the larger average number of seeds which the capsules contain. The difference also between the number of seeds produced by the long-styled and short-styled flowers of the primrose, when both are illegitimately fertilised, is greater than that between the number produced under similar circumstances by the two forms of the cowslip. The long-styled flowers of the primrose when protected from the access of all insects, except such minute ones as Thrips, yield a considerable number of capsules containing on an average 19.2 seeds per capsule; whereas 18 plants of the long-styled cowslip similarly treated did not yield a single seed.