Trans. Nat. Hist. Soc. Northumberland and Durham 5(2): 244-98. (1921)

The Genus Rosa, its Hybridology and other Genetical Problems
J W Heslop Harrison D.SC

Pollination in the Rosae

Investigating as I was the possible effect of cross-pollination in the roses, I was compelled to study the matter in the field, when some very interesting and illuminating evidence was secured. Very early indeed I discovered that, to say the least, pollination in Rosa was conducted under peculiar circumstances. Every morning at 7 a.m. practically every young flower, no matter what its species, provided that its stigmas were mature enough to receive pollen, was already pollinated, and this maturity, since the roses are homogamous, was almost always shown at that hour. Thus it appeared almost certain that, if pollination was effected by insects, it could only be though the action of Noctuidae flying at dusk and dawn, or through Diptera and Hymenoptera busying themselves at daybreak. To determine which was responsible I paid special attention an hour or so after sunset to the blossoms of the day, and to those just ready to burst. At that time, as if by magic, every flower young and old was folded up for the night. Unless then brought about by casual day-fliers like the Noctuids of the genus Miana the agency of moths must be ruled out. There remained then the operations of Diptera and Hymenoptera to be considered. I therefore got up earlier, at 4 a.m. (GMT), before any insects were at work, when I found that even then every newly expanded R. pimpinellifolia had its stigmas powdered with pollen from its own overarching stamens. At the same time those of R. mollis, R. omissa and R. Lintoni were quite untouched. A little later even their anthers dehisced, after which, unless insect guests performed the necessary operation, on the maturing of the innermost stamen whorls they curved over and deposited their precious dust. In many cases indeed even this curving motion as a mode of self-pollination proved superfluous, for almost without exception, as the flowers mature, several stamens are locked between the stigma heads so that their pollen, as it is shed, of necessity falls on the adjacent stigmas. By one method or another, therefore, in default of outside agencies, self-pollination is automatic.

* Unless the shining secretion on the exposed disk of the Eucaninae, Agrestes and Tomentosae is nectar the Rosae do not secrete that substance.

Independent of this definite mechanism for utilising their own pollen, the same effect is frequently brought about in diverse ways by insect visitors. Firstly, every flower bears its own colony of the widely distributed Taeniothrips primulae — an insect, contrary to the indications of its name, very impartial in its predilections. This Thrips rarely leaves its own particular flower, but spends its time in wandering indiscriminately amidst stigmas and anthers, and thus unavoidably carries pollen from the latter to the former. Secondly, insects from elsewhere, whilst in some measure rendering cross-pollination possible, since they are always pollen eaters and not nectar* gatherers, crawl at random from anther to anther, touching the stigmas as they pass, and deposit strange as well as adjacent pollen with the same result as before. Thus, to a very unexpected degree, the roses prove autogamous; that cross-pollination can occur, and in a manner likely to secure hybridity, the following journeys of the bees Bombus pratorum and Andrena trimmerana will give adequate proof.

  1. Journey of a worker Bombus pratorum:— Rosa mollis var. caerulea, R. pimpinellifolia, R. omissa.
  2. Journey of a second worker B. pratorum:— R. mollis type, R. mollis var. caerulea.
  3. Journey of a female Andrena trimmerana:— R. pimpinellifolia, R. mollis var. caerulea, R. mollis type, Saxifraga lingulata, Rubus idaeus, Aquilegia canadensis.

How generously the roses are patronised by insects, and therefore some degree cross-pollination possible, can be determined from the table on pages 272 and 273.

With such a crowd of guests as this, and guests of such cosmopolitan tastes, could one be surprised, even where autogamy and apomixis to a greater or less extent prevail, if hybridity became exceedingly frequent?

Cleistogamy

In such a genus as Rosa one would scarcely anticipate the occurrence of cleistogamous flowers, so little need does there appear for their existence. Their development in Viola spp., Oxalis acetosella, Lamium amplexicaule and, to a less extent, Drosera rotundifolia and D. anglica is quite understandable. With the rapid growth of the surrounding vegetation in spring these species run grave risks of being overwhelmed and thus prevented from receiving the necessary insects. To avoid this contingency this very specialized method of self-fertilisation has been evolved. No such fate seems to threaten Rosa omissa, so that the discovery of a considerable number of cleistogamous flowers in that microgene occasioned great surprise. In general structure these were quite comparable with similar flowers of Viola hirta.

Their ovate lanceolate sepals were perfectly normal, but the involute petals, of the usual shape when straightened out, were reduced in dimensions to 8 mm. by 4 mm. In colour, they were greenish white. Otherwise, both the male and the female organs were quite ordinary. Most significantly, the amount of fully developed pollen attained the surprising figure of 100 per cent. — a contrast with the normal flowers of the same bush which yielded 70 per cent. good grains.

Apomixis

The defective nature of the pollen in many rose microgenes seemed, at first sight, to constitute a very effective barrier to their reproducing their kind, and therefore stimulated enquiries as to how the affected forms did so. To this end bushes known to have produced very defective pollen were marked in July and visited in September. Without exception, these individuals then bore, if possible, a greater supply of hips than their companions. Further, in one instance, a Rosa rubiginosa, the plant grew five miles from another member of the genus. Only one explanation seemed to account satisfactorily for the observed facts, and that was that in Rosa some form of apomixis was prevalent.

To put this to the proof, in the following season certain blossoms on each of the shrubs transferred to the garden were deprived of their stamens (care being taken to remove those clasped between the styles) and enclosed in stout paper bags. The roses thus treated comprised representatives of Rosa mollis, R. mollis var. caerulea, R. omissa, R. rubiginosa var. comosa, R. coriifolia var. Lintoni, R. pimpinellifolia, R. rubiginosa Penzance hybrid Lucy Ashton. In the same year, in June and July, a considerable number of forms were similarly castrated in an unfrequented lane running along an old waggon way abandoned a hundred years ago. Remote from the colliery village as this lane is, too many people use it as a short cut to admit of the possibility of leaving the twigs bearing the experimental flowers exposed in paper bags. I therefore very carefully cut the stigma heads off the mutilated flowers. The bushes so dealt with were R. mollis, R. omissa, R. tomentosa var. sylvestris, R. lutetiana, R. dumetorum, R. coriifolia var. Lintoni and R. glauca var. subcristata. In the garden all of the fruit fell save for those on the two Rubiginosae and certain of R. var. caerulea. On the waggon way matters did not pursue the same course; by August 8th all of the Villosa forms had fallen, and by August 15th the R. tomentosa var. sylvestris and R. var. Lintoni had followed. Only hips of R. dumetorum, R. lutetiana and R. glauca var. subcristata thus remained. A month later, of these, R. lutetiana and R. dumetorum alone persisted, accompanied by a solitary R. glauca. In October the surviving glauca hip had vanished; then, since they were now ripe, the two lots of Eucaninae fruits were removed for dissection. When examined, not a single fruit contained anything save a few useless chaffy scales. In the Rosa caerulea from the garden the fruit contained eight seeds on the average against 20 in the case of those pollinated normally; the two Rubiginosa supplied perfect seeds not differing widely in numbers from those flowers left exposed to chance pollination, self, insect, or otherwise.

The position of the Rosa glauca var. subcristata needs special attention. Unlike what occurred with the Villosae, Tomentosae and Eucaninae, the beheaded fruits began to swell as usual and gave every promise of abundant seeds, as was disclosed when examples were opened for examination. The future wholesale failure was far from being suspected, and it was with considerable dismay I beheld the gradual dropping of the hips. The reason, however, was only too apparent after dissection; on account of the sessile head of stigmas, to ensure their complete removal, I had cut too deeply into the fruit so that decay and insect enemies could work their will unhindered, thus causing its premature fall with its store of half-ripened seeds.

From these experiments we perceive that at least Rosa mollis var. caerulea, R. rubiginosa var. comosa, R. rubiginosa, hybrid Lucy Ashton and R. glauca var. subcristata are apomictical. All the other microgenes tested would seem, at first sight, to depend for their successful seeding on pollination either by means of their own sound microspores or by foreign pollen conveyed by insects. However, experiments carried out during 1920 prove that all our local microgenes examined, save Rosa arvensis and R. pimpinellifolia, are to some extent facultatively apomictical.

Although not submitted to experiment, Rosa Borreri seems likewise to be apomictical. A certain shrub, whose pollen proved wholly defective when put under the microscope in June, was nevertheless decked with a brave array of crimsom globes in October. One of these carried at its apex the unopened flower bud. This seems proof positive that in this particular blossom some type of apomixis had taken place, and suggested that the other fruits originated in the same manner. Of course in this, as in all the other instances, the apomixis may be of the facultative order.

The proof of apomixis existing in Rosa does not rest solely on my work, for Dingler has proved it to occur in an unnamed Rubiginosa form, whilst Lundström demonstrated its existence in varieties of R. glauca and R. coriifolia.

V. LITERATURE CITED.