POLLINATION OF THE BRITISE PRIMULAS. 105 The Pollination of the Rritish Primulas. By MILLERCHRISTY, F.L.8. rRend let December, 1921*.] SYNOPSIS. PAGE I.-Introductory Remarks .................................. 106 II.--Observations on Insect Visitors .......................... 108 111.-The Depths of the Corolla-tubes of the Flowers. ........... 123 1V.--The Tongue-lengths of the lnsects known to visit the Flowers. 1% V.-Summary of Observations on Insect Visitors .............. 126 V1.-Critical Remarks on the Observations .................... 129 VI1.-Conclusion ............................................ 134 I.-Introductory Remarks. FORTYyears or so ago, one often heard or read papers dealing with some special point in connection with the pollination (then called “ fertilization ”) of flowers by insects. At that time, the whole subject mas practically neu, having recently been brought to the front ns a result of the siiperh experi- inental and observational work of Darwin. Since then, the chief problems involved have been solved, largely through the labours of our past-President, Sir John Lubbock (afterwards Lord Avebury), Alfred TV. Bennett, John Scott, the Rev. George Henslow, I. H. Burkill, and others in this country, but mainly through the work of German observers, two of whom have published almosbencyclopaedic works on the subject. Yet there is still much to be learned in detail as to the means by which the flowers of certain plants secure pollination. Of few plants, if of any, is this more true than it is of the British members of the genus Primula. Indeed, the precise means by which the flowers of these plants (especially the Primrose) secure adequate pollination has long been, and still remains, a complete mystery, though there has been much discussion, and close attention has been given to it by many observers, including Darwin i,T. R. Archer Briggs $, Hermann Miiller 0, myself 11, * Revised slightly Nay 1922. t Journ. Linn. SOC.,Bot. x. (1869), pp. 437-454, aud ‘Differeut Foriiis of Plowers on Plnnts of the same Species,’ pp. 23, 36, Cc. (1877). $ Journ. of Bot. (Seemanu’s), viii. (1870), pp. 190-191 ; Trms. Plpiouth Instit. iv. (1872), pp. 180-190; and ‘ Flora of Plymouth,’ p. 279 (1880). 5 ‘Die Befruchtung der Blumen durch Insekten,’ pp. 346-347 (Leipzig, 18i3), md ‘The Fertilization of Flowera,’ transl. and ed. by Prof. D’hrcy W. Thompson, pp. 383-386 (Loud., 1883). )I Seeport., p. 107 n. 106 MR. MILLER CHRI8TY ON THE Prof. Ilugero Cobelli *, John French?, Alexander H. Gibson $, G. F. Scott-Elliot 5, I. H. Burkill 11, Dr. Paul Knuth q, Rev. E. Bell **, Prof. F. E. Weiss tt, Prof. G. S. Boulger $$, Miss Mary L. Armitt $8, the Rev. E. T. Daubeny [I 11, E. G. HighfieldVl, and A. A Dallman ***. One of these devoted most of a large part of a fair-sized volume to a discussion of the problem, but without, I think, contributing anything material to its solution ; nor does the latest contribution to the subject (that of Mr. Dallman), though painstaking, carry it much further, so far as I can see. That so much observation and discussion should have produced so little result is not altogether surprising; for the problem presents (as will be found) certain unusual complications. There are some plants whose flowers exhibit a highly complicated floral mechanism (the Orchids, for example) ; yet the manner of their pollination is fairly obvious. On the other hand, the dimorphic heterostyled flowers of the Primulas exhibit a floral mechanism which, though highly ingenious, is comparatively simple ttt ; yet (as has been stated) the precise mauner of their pollination bas defied investigation and remains to this day unexplained. It may be doubted, indeed, if there is any problem of the kind more pnzsling than that which has been called “ the Mystery of the Primram.” Three species of the genus Primula are immensely abundant in Britain, though one of them is so in one district only. All three species are also very common and very widely distributed on the Continent. They are :- * Verhandl. der I(. K. 2001.-bot. Geaellsch. in Wien, xlii. (189!3), pp. 73-18. t &sex Naturalist, v. (1891), pp. 120-124. $ Trans. Bot. SOC.of Edinb. xir. (1893)’ p. 166. 5 Vora of Dumfriesshire, p. 114 (1896). I] Jourti. of Bat. XXXV. (1897), p. 186. Handh. der Bliitenbiologie, ii. (1899), pp. 308-319 (Leipzig, 3 vols., 1898-1905),and Handb. of Flower Pollination, trans]. by Prof. J. R. Ainsworth Davis, iii. pp. 64-76 (Oxford, 3 vole., 1906-09). ** ‘ The Primrose and Darwinism,’ by a Field Naturalist ’ [i.c., E. B.], (Lond., 1902). i-t New Phgtologist, ii. (1903), pp. 99-106; iii. (1904), pp. 168-171; and Nature Notes, xv. (l904), pp. 103-106. $1Nature Notes, xv. (1904), pp. 84-86. 55 New Phgtolngist, iii. (1904), p. 170. 11 u Nature Notes, xvi. (1906), pp. 116 & 136-137. 911 Knowledge, xxxix. (1916) pp. 113-117. *** Journ. of Botany, lix. (1921), pp. 310322 & 337546. ttt It was, however, Darwin’s opinion that tri-morphic heterostyled flowers, such a8 those of Lytltrum Sulicuriu, present ‘‘ a more extranrdinrry and complicated arrangement of the reproductive system tlrnn can be found in any other organic being8 ” (see his Introd. Note to Miiller’s ‘Fertilization of Flowern,’ p. ix). POLLINATlON OF THE BRITISE PRIMULAS. 107 (J) The Primrose (P.vulgaris HudAon, vel acnuliu Linn.), (2) The Cowslip (P.iwrh Linn., vel o$Cinalis Jacquin), (3) The True Oxlip (P.elatior Jacqnin)*. On the distribution, distinctive features, and other peculiarities of these three species, as observable in Britain, I have already published two lengthy papers t. The question as to how the pollination of our Prirnnlas is effected was first raised by Darwin. Between 1862 and 1868, he read before the Linnean Society five papers $, in which ho pointed out clearly and conclusively the significance of heterostylism, especially as exemplified in the Primulas 5. Later, these papers, collected and revised, appeared in the form of a work dealing comprehensively with the whole subject, as then known 11. In the course of his investigations, Darwin naturally made observations to ascertain what insects effect the pollination of the Primrose and the Cowslip7 ; and the results of these observations require notice at the outset. Of the Primrose, he wrote, in 1869 **, that it “ is never visited (and I speak after many years’ observation) by the larger humble bees, and only rarely by smaller kinds : hence its fertilization depends almost exclusively on moths”- a statement the first portion of which is certainly incorrect, as Darwin him- self afterwards came to see. Much newer the truth, though still somewhat short of it, is his final statement tt- It is surprising how rarely insects can be seen duriug the day visitiug tho flowers, but I have ocoasionally observed small kinds of bees at work. I suppose, therefore, that they are commonly fertilised by nocturnal Lepidoptera. * As all three are very well known by their vulgar name@,I use thcse in speaking of them hereafter. There are also two other British species of the genus-P. futinosa Linn. and P. scotica Hooker; but of these, though they are covered by my title, I have nothing to say. Nor have I anything to say of the various hybrids into which all three species enter, ench with both the others: namely P. vulyaris x verh (the Common Oxlip”), P. vulgariux elatior, and P. elatior x ueiis. t (1) Trnns. Essex Field Club, iii. (1884), pp. 148-211, nnd (2) Journ. Linn. Soc., Bot. rxxiii. (1897), pp. 172-201. $ All afterwards published in its Journal (Rot.), vola. vi., vii., and x. 5 “Hetarostyliem” ia the term he used, and it is preferable to, because more definite thm, “heteromorphism,” a term suggested by Asa Gray and used by Lubbock and some later writers. 11 ‘The Different Forms of Flowers on Plants of the same SFeciee ’ (London, So, 1877). 7 Darwin records no similar observations on the Oxlip, a plant he probably never saw growing wild. ** See Journ. Linn. SOC.,Bot. x. (1869), p. 438. j-t Forms of Flowers,’ p. 36 (1877). 108 MR. MlLLER CHRISTY ON TEE Of the Cowslip, Darwin wrote in 1862 Q :- What insects habitually visit Cowslips, as is absolutely necessary for their regular fertility, I do not know. I have often watched them, but perhaps not long enough ; and only four times I have seen humble-bees visiting them. One of these bees was gathering pollen from shorbstyled flowers alone ; another had bitten holes through the corolla; and neither of these would have been effective in the act of fertilization. Two others were sucking long-styled plants. Here, again, we find Darwin’s earliest statement incomplete‘ and valueless. Later, he extended and amended it several times t. His final statement 2 is to the effect that- The flowers of the cowslip and of the other speciee of t,he genus secrete plenty of nectar ; and I have often seen humble-bees, especially B. hortorum and mu~corum, sucking the formor in a proper manner, though they sometimes bite holes through the corolla. No doubt moths likewise visit the flowers, as one of my sons caught Cucullia verbasci in the act 6. These observations of Darwin’s were (for him) remarkably incomplete ; for, as a fact, insects visit the flowers of our Primulas much more frequently than he realized. In a problem of such complexity, there is, I submit, no more hopeful means of reaching a solution than that of summarizing and reviewing in detail all the evidence available, in the hope that one may tberebj guide some other observer into the right track ; and this, precisely, is my present aim. 11.-Observations on Insect Visitors. In three Tables which follow (each relating to a particular one of the three species of Priinula concerned), I set forth in detail the results of my own personal observations, extending over rather more than forty years, on the different species of insects accustomed to visit each species.