THE IMPORTANCE OF SELECTION
IN VEGETATIVE PROPAGATION
W. R. BUTTENSHAW, M.A., B.Sc.,
Scientific Assistant on the staff of the Imperial Department of Agriculture
The question whether plants propagated by vegetative processes could be improved or permanently modified by selection has often been debated. In most of the processes of vegetative propagation the new growth is merely an extension of the growth of the parent, and, consequently, the new plant produced in this way is much like its parent. But since even among the different branches of one tree there can be variation—sometimes sufficiently obvious to become a 'bud sport'—it is easy to imagine that cuttings produced from the same parent may develop considerable variation.
It is well recognized by gardeners that a new individual produced from a cutting possesses certain characters which may or may not differ from all other similar parts of the parent plant; in other words, hardly any two cuttings possess exactly the same characters; that it is also necessary to make use of careful selection when propagating plants by a vegetative process is therefore apparent.
A very complete series of experiments has been carried out by Messrs. B. T. Galloway and P. H. Dorset, of the Division of Vegetable Pathology of the C. S. Department of Agriculture, to determine to what extent violet plants could be improved in productiveness, vigour, and ability to resist disease by a careful selection of cuttings. A detailed account of these experiments is given by Mr. Herbert J. Webber in the Yearbook of the U.S. Department of Agriculture for 1898 (pp. 373-5). He says: 'The results already show that productiveness is remarkably increased, and they also clearly demonstrate that violet cuttings can gradually be improved by a continuous selection of the cuttings used in propagation and of the plants from which these are obtained. The method consists in selecting a number of the finest-looking plants before they begin to bloom, placing beside each a stake to which a blank tag is attached, and carefully recording on each tag the daily pick of saleable flowers from the plant, so that at the end of the season the number of flowers produced by each plant is known. The cuttings for the ensuing year are taken only from the plants producing the greatest yields, and which are known from continual observation through the season to be desirable in other ways. The pedigree cuttings thus obtained are again subjected to selection, and only those which root well and form good, vigorous, young plants are finally used.'
The following is an interesting example of the nature of the results obtained: Five plants were selected from a plant which, in the previous season, had yielded eighty-five flowers. Three out of these five gave a much greater yield than the parent (127, 109, and 103,) the remaining two gave eighty-two and eighty-four, respectively. The average yield of the five plants is thus 101, or sixteen flowers more than that of the parent.
Selection in vegetative propagation is also of assistance in producing a healthy strain of plants. Webber gives an account of experiments with the Ripley Queen pine-apple, which is liable to a disease which causes it to 'go blind,' that is, advance to the end of its growing period and sucker from below without fruiting. The experiment consisted in planting in one bed suckers from diseased plants and in an adjoining bed suckers from apparently healthy plants. In the former bed, eighteen months later, he found that 63 per cent. had contracted the disease, while in the other bed slightly less than 4 per cent. showed the disease. This being the result of but one selection, it would seem probable that the disease might be completely controlled by a continuous selection of suckers from healthy plants.
Another interesting illustration of the modifications obtained by time careful and continued selection of vegetative parts—one of particular interest in West Indian agriculture—is the breeding out of thorns from citrus trees by bud selection. To quote again from Mr. Webber's article: 'Seedling oranges and lemons are almost invariably very thorny, but nevertheless the majority of the standard varieties cultivated are now largely thornless, owing, it is said, to the continuous selection of buds from thornless branches. According to the testimony of orange nursery-men, it is quite certain that thorns can be bred out in this way in every case, and usually to do so requires but three or four bud generations. It is probable, in the case of other fruit trees, that by selecting buds or cuttings from branches that are thornless, or which have fewer thorns than usual, the thorns could be entirely bred out, or at least the greater number reduced.'
Writing in the Bulletin of the Botanical Department of Jamaica for November 1900 on the subject of budding orange trees. Mr. W. Cradwick says: 'Buds with thorns attached should not be used; they do not grow so readily, and, if they grow, result in a tree on which long thorns will be one of the chief features. A tree grown at Hope from a bud with thorns 1 1/4 inches in length attached to it produced thorns over 8 inches in length.'
In this connexion it may be useful to review briefly the interesting series of experiments in the chemical selection of sugar-cane conducted by Dr. Watts at Antigua:—
'The object of the experiment is to ascertain whether the saccharine content of the sugar-cane can be effected by selection of cuttings.' These experiments consist in selecting two series of canes— (1) canes rich in sucrose, called 'High' canes, and (2) canes poor in sucrose, called 'Low' canes. The experiments have now been in progress for four years. Each year the ten richest canes have been selected from the high plot and the ten poorest front the low plot. This represents an attempt to obtain two divergent series— 'one tending to increased richness, and the other to decreased richness.' ln making the selection, the canes are examined by cutting off the basal portion in the middle of the fifth internode from the base, crushing this basal portion in the Chatanooga mill, and determining the amount of sugar by means of the polariscope in the sample of juice so obtained.
The results so far attained are as follows:—
|Pounds of sucrose per gallon||1900||1901||1902||1903||1904|
|Difference on canes planted||.597||.326||.985||.463||.876|
|Difference on canes reaped||.020||.218||.093||.199|
The difference on canes planted in 1900 and 1004, in pounds of sucrose per gallon, has increased from .597 to .876, an increase of 46 per cent. Similarly, the difference on canes reaped in 1901 and 1904 has increased from .020 to .199.
In considering this question it is essential that two points should be clearly distinguished. Briefly put, it is the difference between the maintenance of a 'sport' and the gradual improvement by selection. My meaning will he made clear by another reference to the case of budding out thorns from citrus plants. If a bud is taken from a spineless branch and the resulting tree comes spineless, then we may have simply perpetuated a sport. But can we rely upon all such buds being spineless? This would appear not to be the case, for Mr. Webber says: 'Standard varieties are now largely thornless;' and again, 'it is quite certain that thorns can be bred out in this way in every case, and usually to do so requires but three or four generations.' It is evident that we have here a gradual process, that the variations between spineless and spiny buds are not uniform but differ in degree, and, lastly, that continued selection is necessary before the spine can be eliminated. The same fact is brought out—but in the contrary direction—in the instance cited by Mr. Cradwick, where a tree grown from a bud [with thorns] 1 1/4 inches long attached to it produced thorns over 8 inches long; it is obvious that it would be possible here also to produce two divergent series of oranges—one becoming less and less spiny and the other tending to produce longer and longer thorns.