Journal of the Royal Horticultural Society 24: 90-126 (1900)
Notes on some experiments in hybridisation and cross-breeding

Partial Prepotency
C C Hurst


Those who have studied hybrids between distinct species must be impressed with the undoubted inheritance of specific characters.

Varietal characters, while perhaps of more practical importance, are yet so indefinable, so uncertain, and so fleeting that in the second generation they are with difficulty traced at all. On the other hand, specific characters are more definable, more certain, and more lasting, and can be traced through several generations. For instance, in Paphiopedilum x triumphans, a hybrid of the third generation raised by M. Jules Hye-Leysen, of Ghent, the crimson veining in the upper sepal of the flower can be traced through the parent, P. x oenanthum superbum, and the grandparent, P. x Harrisianum, back to the great-grandparent, P. barbatum.

In studying the inheritance of specific characters, I have found it a great advantage to take up a special group of plants and to study their characters as carefully and minutely as a monographer would do, and in this way I have been able to follow the inheritance of specific characters much more easily than otherwise I could have hoped to do. In following out this idea I have chosen the orchideous genus Paphiopedilum (Pfitz), better known in gardens as Cypripedium, partly because hybridisation has been carried farther in this genus than in many others, and partly because I have the good fortune to have a large collection of living hybrids and their parents of this genus under my own observation from day to day.

The limits of this paper will not allow fully detailed observations, but a few condensed analyses of some primary hybrid Paphiopedilums will well illustrate the inheritance of specific characters. In these cases practically the whole plant has been analysed from living specimens, twenty points in all being taken into consideration:

(1) The habit of growth; (2) the habit of flowering; the form or shape of the (3) leaves, (4) scape, (5) bract, (6) ovary, (7) upper sepal, (8) lower sepal, (9) petals, (10) lip or slipper, (11) staminode; the colour of the (12) leaves, (13) scape, (14) bract, (15) ovary, (16) upper sepal, (17) lower sepal, (18) petals, (19) lip or slipper, (20) staminode.

Each of these parts or organs of the hybrid has been compared with the same part of each of the parent species. Each part is then classed in relation to the two parents, either (a) in the ratio as 1:1, which represents the part as fairly intermediate between the two parents; or (b) in the ratio as 3:2, which represents one parent to be slightly predominant in that particular part; or (c) in the ratio as 2:1, showing the decided prepotency of one parent in that part; or (d) in the ratio as 3:1, showing the very large prepotency of one parent in that part. In this way the twenty parts are classified, and when the various figures are added together one can see at a glance the total ratio of one parent to the other in the hybrid. In the following condensed analyses I have ignored, for the sake of simplicity, all the ratios as 1:1 and also those as 3:2, classing them as intermediate or thereabouts, and only showing the undoubted prepotencies of either parent in the ratios as 2:1 and over. At the end of each are given the full ratios of the plant as a whole, as originally analysed, with the corresponding percentages of the predominant parent, for the sake of comparison. (For additional instances, see also under the heading of "Variation of Primary Hybrids.")

(1) Paphiopedilum x Winnianum, a hybrid raised by Messrs. Veitch, of Chelsea, out of P. villosum (Pfitz) by P. Druryi (Pfitz). In form, the parent P. Druryi is prepotent in habit of growth, leaves, scape, and lower sepal; while P. villosum predominates in habit of flowering, petals, and staminode; in all, favouring P. Druryi as 18 : 17. In colour, P. Druryi preponderates in the leaves, lower sepal, and staminode; while P. villosum prevails in the petals ; altogether favouring P. Druryi as 16 : 15. The whole plant in form and colour favouring P. Druryi as 34 : 32, or 51.5 per cent.

(2) P. x Swinburnei, a hybrid raised by Messrs. Heath, of Cheltenham, out of P. insigne (Pfitz) by P. Argus (Pfitz). In form, the parent P. insigne is prepotent in bract, ovary, and upper sepal; while P. Argus predominates in the lip; in all, favouring P. insigne as 24 : 23. In colour, P. Argus predominates in lower sepal, petals, and lip, altogether favouring that parent as 19 : 15. The whole plant, in form and colour favouring P. Argus as 42 : 39, or 51.8 per cent. (Fig. 16.)

(3) P. x Ceres Medea, a hybrid between P. Spicerianum (Pfitz) and P. hirsutissimum (Pfitz), raised by Mr. Latham, of Birmingham. In form, the parent P. Spicerianum is prepotent in the upper sepal, while P. hirsutissimum predominates in the leaves, ovary, and staminode; in all, favouring P. hirsutissimum as 20 : 19. In colour, P. Spicerianum preponderates in the leaves, bract, upper sepal, and staminode; while P. hirsutissimum prevails in the scape and petals, altogether favouring P. hirsutissimum as 16 : 15. The whole plant, in form and colour, favouring P. hirsutissimum as 36 : 34, or 51.4 per cent. (Fig. 17.)

Fig. 15. P. x Winnianum Fig. 16. P. Swinburnei Fig. 17. P. Ceres Medea
Hybrid Paphiodelums

Hybrids of the first generation between the same pair of species are found to have a certain specific likeness, yet at the same time they differ one from the other in varietal characters.

Sex, per se, does not seem to have any influence in the variation of hybrids in the Orchideae (owing possibly to their being hermaphrodite by nature), the same varieties occurring both in the reverse and obverse crosses; indeed, in several cases recorded, the progeny of the reverse cross and that of the original one have proved to be exactly the same. As we have seen in the inheritance of varietal characters, when a different variety is used as a parent, the result tends to be different, the variation generally corresponding with that of the variety used. But the variation of first hybrids extends beyond the differences caused by using different varieties as parents, because we often get considerable variation among hybrids raised from the same seed-capsule.

How then is this variation to be explained? A careful analysis of hybrids of Paphiopedilum seems to give a clue to this problem, as the following condensed analyses show. (Compare also those given under the heading of "The Inheritance of Specific Characters.")

pp. 110-111    

These [Paphiopedilum] hybrids, as a whole, are fairly intermediate between their two parents, yet there is in most cases a local predominance of one parent or the other in one part or another of the hybrid. This applies equally either to form or colour.

When several hybrids from the same pair of species are compared together, this variation of the parts, or “Partial Prepotency,” as I propose to call it, becomes even more apparent and more diverse. For example, in three hybrids raised from the same parents, in the first, the pollen-parent may predominate in form in a certain part; in the second, the seed-parent may prevail in that part; while in the third, that part may be fairly intermediate between both parents; while in regard to colour, these conditions may be exactly reversed. But this only includes one part of the hybrid, and the same law applies equally to every one of the parts so that when the changes are rung on twenty or more different parts by the two parents in both form and colour, we can well understand the many possibilities of variation in hybrids of the same parentage; and I venture to suggest that this law of Partial Prepotency, founded on actual facts observed in hybrids of Paphiopedilum, may perhaps throw some light on the question of variation in offspring of the same parents. Yet, notwithstanding this variation in the parts, it is a remarkable fact that in primary hybrids the whole plant taken together is fairly intermediate between the two parents, the balance of power being well maintained in the whole.

The greatest extreme observed by me, out of many cases in Paphiopedilum, has been 58.6 per cent. of one parent, against 42.4 per cent. of the other, the great majority being approximately 50 per cent. of each parent. This hardly coincides with the popular belief that some hybrids resemble one parent, while others resemble the other; but this may be due simply to superficial observation, for where conspicuous parts lean towards one parent, the casual observer might easily be deceived, not noticing the inconspicuous parts which compensate for this by leaning towards the other parent.

CybeRose note: By the time I read this paper, I was already calling this sort of inheritance "Elective Expression", with a nod to Dr. Prosper Lucas's "élection", described in his book, Traité philosophique et physiologique de l'hérédité naturelle, vol 2 (1850).

MacFarlane also compared hybrids with their parents, examining microscopic details.

Macfarlane: Microscopic Analysis of Hybrids (1890)
Macfarlane: Flowering Period and Vigor of Hybrids (1891)
Macfarlane: Minute Structure of Plant Hybrids (1892)