Report of the
Conference on Genetics (1903)
THE CROSS‑BREEDING AND HYBRIDISATION OF PEAS AND OF HARDY FRUIT
W. LAXTON, F.R.H.S.
THE object of cross‑breeding, from a practical and commercial point of view, may be described as an effort to eliminate the bad, and intensify the good characters of a plant. With this object in view we naturally select as parents two plants bearing, in a greater degree than their neighbours, the features we wish to intensify. Having made the crosses and got our seedlings up, we expect to find some bearing the desired characters in an improved form, but in actual practice we are often greatly disappointed, and feel inclined to throw them all away. We have, however, been taught by Mendel to be patient, and not to expect too much in the first generation; if Mendel has only done this he has done more good than one at first realises, for he has saved from destruction many latent improvements of future generations, and encouraged the cross‑breeder to proceed with his work instead of abandoning it in disappointment and disgust. But we must not expect too much from Mendelism. The progress of improvement in annuals such as the pea will show us that real improvements are slow, and the combining of particular characters in one plant takes many years of careful work, even with a knowledge of Mendel's theories. The late Thomas Laxton, who may be said to be the follower of Knight and McLean in the cross‑breeding of Pisum, devoted many years to working on the pea, and his principal breaks which stand after all these years are not many in number. His work was not conducted in the dark, as he knew long before others that the pea was a self‑fertilising flower, requiring to be emasculated in the early bud state, and that breaks were not to be expected in the first generation, although he had not worked out any law as to the ratio in the second and third generations, as Mendel has done. Great as are the benefits arising from the knowledge of Mendel's law, we find that we cannot arrive at any desired result without much labour and patience, and still something must he left to chance in the combining of many desirable characters in one plant, and great quantities of crosses will have to be made before we attain the end. Mr. Bateson, Mr. Punnett, Mr. Hurst, and others have worked on most careful lines, and can tell us how many generations will have to be raised if we desire to combine, say, five or six characters which are known to be either dominant or recessive, in any particular plant.
In hardy fruits, such as the apple, pear, plum, or strawberry, we have been working with impure strains, and have had to trust to chance combinations of desirable characters, hoping by crossing from our best crossbred seedlings to get the greatest number of breaks. Unfortunately, many of these are combinations of three or four characters with an important one missing. These constitute failures, as in a commercially valuable fruit we have to combine many good points. Take, for instance, an apple‑‑(1) flavour; (2) size; (3) colour; (4) earliness or lateness; (5) cropping qualities; (6) constitution. Chance may favour the hybridist and he may hit the mark, in combining four or five out of the six, but the one "missing" character condemns the product as useless. Therefore, as we have not any guide as to these characters being either dominant or recessive in their generation, we have to make very many crosses before we succeed in combining them all in one plant. In the whole of my experience in cross‑breeding, I have never been able to exactly reproduce any one of our seedlings, in all its characters, by re-effecting the same cross. For instance, 'Royal Sovereign' strawberry was raised by crossing 'Noble' x 'King of the Earlies,' and although this cross has been effected many times since, we have never reproduced just 'Royal Sovereign.'
THE CROSS‑FERTILISATION OF THE PEA.
The artificial cross‑fertilisation of the pea, like that of most other plants, is very easily effected. It is not the actual crossing that is expensive and laborious, but it is in the after part that the labour and expense accumulate in the sowing and selecting, and re‑sowing and re‑selecting, and afterwards thoroughly fixing the variety.
In practice, the bloom is opened in the very young bud stage, emasculated, and pollinated in the usual way. We do not find it necessary to protect each individual bloom from insects if the pollen is applied in the early stages, as the pistil is already well protected from outside interference by the pollen grains applied. The pea, being what is termed a self‑fertiliser, is almost, if not perfectly, immune from insect interference. Hence the practicability of growing side by side various types of peas, and securing a true stock on re‑sowing. The late Mr. Laxton was one of the first experimentalists to prove this, and to point the fact out to Darwin. After having effected the various crosses desired, and harvested the pods, the seed is sown in the following spring. In the first generation no deviation from the dominant seed‑parent is noticed or expected. The second season the produce is again sown, and here we look for the breaks desired. If, of the parents of the cross, one is tall and one dwarf, we find nearly all the produce will be tall, but a few dwarfs and semi‑dwarfs are also noticed, and are selected from the others. The appearance of a very large proportion of tall peas and a few dwarfs is satisfactorily explained by Mendel's theory, and how far this law applies to semi‑dwarfs which are also found (that is, intermediate in height to the two parents) I am at present unable to follow or explain. The produce of this cross is again sown the following season. All the dwarfs, semi‑dwarfs, and talls being sown separately, we find in this generation a further splitting‑up of the types, dwarfs still appearing amongst the tall selections in the proportions of the second generation, but the dwarfs and semi‑dwarfs do not further break away in height, but come true. Therefore, we have to grow and select each cross through at least three generations before any fixed and definite type can be secured. Afterwards, the selected seedlings, if any‑in many cases they are all useless‑are further sown and tested, and we find "rogues," or false peas, still appearing for many generations, mostly of a very wild or common type. It is curious to note that in the varieties of cultivated peas this wild type appears to be the same in all, and occurs continually as a "rogue." It is easily distinguished by its "vetch‑like" growth and short curved pods. In further reference to Mendel's theory, we find from examination that "blunt" or "square‑ended" pods are dominant; that is, if a pointed pod of the 'Duke of Albany' type is crossed with a square-ended pod of the 'Ne Plus Ultra' type, the result will give a dominant square‑ended pod in Mendelian proportions. Early‑ripening and late‑ripening varieties crossed together do not give some earlies and some lates, and some intermediate in season, but nearly all the produce will be late in ripening: here again Mendel's law applies. Also if two early varieties are used in the cross, one would naturally expect the product to consist in the main of early‑ripening seedlings, but in actual practice we find that this is not so; and for this reason it is necessary, if any useful results are to be obtained, to effect many crosses.
To the fact that we use impure, not fixed strains, in our crosses‑that is, seedlings of the second and third generations that are not fixed typesI attribute our deviations from Mendel's law, and believe that it is from this fact of mixing impure bloods together that the greatest breaks are to be looked for. In peas, again, we do not find that the same cross effected several times gives each time the same result. For example, take 'Gradus,' which was a seedling from 'Earliest of All' x 'Duke of Albany' . This cross has since been made many times, but we have not yet found a pea exactly like 'Gradus ' in any of the crosses.
The attempted cross‑fertilisation of Lathyrus odoratus with various perennial species such as L. latifolius, L. grandiflorus, L. pubescens, has so far proved a failure with us, the pollen only being sufficiently potent to irritate the ovary without the formation of fertile seed.
Our work amongst these has extended over a great number of years, the late Mr. Laxton having received his first certificate for a new strawberry from the Royal Horticultural Society as far back as 1866. Since then we have duly effected and actually sown some 1,500 crosses. Taking an average of some twenty seedling plants from each cross, at least 30,000 seedlings have passed through our hands during the last fifteen years. The actual cross‑fertilisation of the strawberry is very simple. The bloom selected is opened in the bud stage and the anthers cut out, the pollen from the selected male parent is at once applied, and again twice after the bloom has expanded. This work is conducted under glass, and all insects as far as practicable excluded by means of tiffany fastened in front of all doors, ventilators, &c. The objects to be sought in crossing and raising seedling strawberries from a commercial point of view are many amongst the chief being;
The latter points many market growers will hold as constituting the blue blood of the strawberry, while on the other hand private gardeners will put quality in the foreground, as strawberries are grown to be eaten as well as to be looked at and be sold. This adds to the necessity for procuring a sufficient number of differing varieties so as to provide for each particular requirement. We ourselves have perhaps been foolish—looking for the philosophers' stone—in seeking to blend all the desired qualities in one. I need hardly say that this happy goal has not yet been reached, and the pleasure may yet be looked forward to by other workers in the field of strawberry raising. But, to be practical, what is really wanted are early, main crop, and later varieties, having goodsized, high‑flavoured fruits, with a firm exterior, the colour of a bright glossy‑scarlet. The conical or heart‑shaped form may perhaps find most favour, but the shape should be regular, the plant hardy, moderately vigorous and sturdy, and fairly productive of runners, a stout footstalk carrying about ten or twelve even and regular‑sized fruits, held above but not far from the ground; the fruits of good and distinct flavour, it not being necessary that all should assimilate in this respect, variety being desirable to suit varying tastes; and lastly, if these qualities can be imparted to fruits suitable for forcing, a material gain would be secured.
These are the points needful in a commercially useful strawberry but just how they are to be obtained is quite a different matter; for if only one of these essential points be wanting, the plant may have to be discarded as worthless. For instance, size and colour, without firmness and flavour, would brand the seedling as useless. The greatest breaks we have yet secured have been through using as parents selected seedlings, having most of the necessary points combined, but lacking perhaps one or two of the essential particulars enumerated above.
The ' Bedford Champion' was raised from a compound cross as follows :—
From this compound crossing of blood we believe the greatest breaks in this and many other fruits may be expected. It is curious to note the great variations in the seedlings from the selfsame cross, repeated several times. Out of several hundreds resulting from the cross, no two will be exactly alike.
In our experiments in hybridising other hardy fruits, we have been successful in raising some rather interesting plants.
(1) We have a hybrid raised from a Japanese Plum x Peach. This gives what appears to be a combination of characters in foliage, at any rate, intermediate between the two parents, and we rather anxiously await the fruit (fig. 131).
(2) Japanese Plum x Moorpark Apricot gives us also what appears to be a combination of characters of the two species (fig. 130).
(3) Greengage Plum x Moorpark Apricot also with the combination of characters of both parents in the foliage.
The modus operandi in crossing the above is similar to the strawberry: that is, the anthem of the bloom to be crossed are cut out whilst yet the bloom is in the bud stage, the pollen of the selected male parent being immediately applied, and once or twice afterwards; all insects being as far as possible excluded.
In the case of stone fruits such as peaches, plums, &c., the pulp should be removed from the stone immediately the fruit is ripe, otherwise if it is allowed to remain we find the kernel inside the stone will not keep, but soon becomes mouldy and rotten. The kernel is sown at once, and germinates the following spring. It is then budded on a congenial stock, and in the following season is potted up. From the time the cross is effected to the fruiting stage takes about five or six years.
The Logan‑berry, an American fruit, said to be a hybrid between the raspberry and the blackberry (which parentage we feel is doubtful), has been recrossed again with various English Raspberries in order to secure, if possible, the elimination of the hard core, so objectionable in the Logan‑berry, and at the same time to improve the flavour of the fruit. In this we have entirely succeeded, having selected a seedling bearing the characters of the Logan‑berry, except that the fruit is an enlarged form of 'Superlative' Raspberry.
PYRUS JAPONICA CROSSES.
We have attempted many crosses between Pyrus japonica and various cultivated forms of Pyrus communis, the object being to secure, if possible, a red‑flowered pear. At present our seedlings are not old enough to bloom, but appearances suggest that the cross has been effective.
APPLES AND PLUMS.
We have made many crosses between all the best varieties, but at present have not fruited many, and as good varieties are so numerous we are resolved to destroy all that are not improvements. We find that the seedlings (the result of crossing some of our best varieties of apples) show a great tendency to revert to a "wild" and "spiny" growth resembling the crab in habit; probably this arises from the "spiny" habit of growth being a "dominant" character, and we may find that the smaller number of better growth are true recessives in this character in the second generation. The great difficulty in following out the Mendelian characters in these hardy fruits arises from the fact that at least five years must elapse from sowing to fruiting each generation; and from the fact that fruit trees occupy much space, the expense of raising twenty or thirty seedlings from each seedling in the second generation to test Mendelian characters may be imagined.
The same remarks apply generally to Plums. We have, however, fruited the following crosses, and give a brief description of the results :—
Victoria x Sultan = Yellow plum, something like 'Jefferson,' of good flavour.
Sultan x Early Prolific = Shape and colour of 'Prince of Wales,' but larger, and of good flavour.
Monarch x Pershore = Small yellow plum; this looked like a good cross "on paper"; but the seedling is quite valueless.
Greengage x Sultan = Small, black plum; valueless.
Grand Duke x Czar = Large, very early, black, and of good flavour.
Finally, I must say that considering the time, labour, and expense involved, the raising of new and improved varieties is, for a nurseryman, a slow and disappointing process, and commercially unprofitable, and I shall hail with delight the time when the workers in Mendel's footsteps can direct us how to attain the desired result by a less laborious, and quicker route.