Principles and Methods
Ivan Michurin

Chapter 3

DISTANT (INTERSPECIFIC AND INTERGENERIC) CROSSINGS.
THE METHOD OF VEGETATIVE APPROXIMATION

The erroneous assertion made by botanists of the past that the crossing of plants belonging to different species and genera cannot be applied and that hybrids thus obtained are always barren for many years deprived me of the possibility of introducing hybridization on a larger scale.

Only after having accidentally come across interspecific and intergeneric hybrids of cultivated plants among my F2 hybrid seedlings did I turn to artificial crossings of plants belonging to different species and genera. Although the success of this work was slow in coming as compared to the usual crossing of plant variations belonging to one species, nevertheless the results obtained were of considerable value.

Furthermore, in the course of the work the following points became evident, namely, that:

1) interspecific crossings can be effected with greater facility when the female parent is a young hybrid in its first blossoming and not a pure species type;

   1) That is, cuttings of hybrids obtained from the crossing of two varieties belonging to one species; the hybrids must be of recent origin, and such as had not yet fruited; the cuttings from our old varieties of fruit trees should not be used.
    2) Not all cuttings are able to take root. For example, some of the varieties of pear have no attraction for quince, and the reverse; or certain hybrids of almond and cherry fail to join well with plums and vice versa. But in our nursery there are cases of good coalescence even between plants belonging to different families; for example, my closest assistant P. N. Yakovlev has been able to induce coalescence between a lemon and a hybrid seedling of the pear Michurin Beurré Zimnaya.

2) a method which I called "preliminary vegetative approximation" is of great assistance in such crossings; it consists in the following: a few cuttings1 are taken from one-year-old hybrid seedlings and are whipgrafted on to the branches of the crown of a mature tree belonging to a different species or genus, as for example: a pear on an apple, a mountain ash on a pear, a quince on a pear, an almond, apricot, or peach on a plum, etc. Only a few of the grafted cuttings, particularly in stone cultures, join with the stock.2 During the following five—six years such cuttings continue to develop under the constant influence of the work effected by the entire leaf system of the stock and gradually begin to change partially in structure up to the time of blossoming; this facilitates the subsequent crossing.

It has further been ascertained that the sterility of interspecific hybrids is not constant in all cases. On the contrary, there are many hybrids which may not give seeds capable of germinating during their first fruit bearings, but in the years following, by gradually improving their structure, it is possible to obtain completely germinable seeds.

I shall cite an example of an interspecific, hybrid between the yellow lily (Lilium Szovitsianum Hort.) and the red (Lilium Thunbergianum Roezl. & Schult.). The hybrid which I named the Fialkovaya Lilia (Orchid Lily) because of its beautiful purple flowers and its orchidlike scent during the first two years of blossoming failed to produce seed balls; on the third and fourth years seed balls appeared, but with empty seeds which of course failed to germinate. Only in the seventh year did the plants begin to produce seeds that would partly germinate.


Reinette Bergamotte

The same was observed when planting the seeds of a black hybrid mountain ash, obtained from the cross of Sorbus melanocarpa ♂ X Sorbus aucuparia L.. For seven-eight years from about a thousand seeds of this hybrid only one or two seedlings would arise; but in 1921 mass germination was suddenly obtained. Among the seedlings quite a number considerably varied in structure.

Furthermore, the same may be said of the vegetative hybrid between an apple and a pear; as a result an excellent new variety of apple was obtained which I named Reinette Bergamotte.

Then again, in some cases the sterility of certain hybrids was eliminated. Thus, the hybrid between Prunus Padus Maackii X Prunus Cerasus blossomed but failed to fruit. When it was budded on to a sweet cherry stock with the aim of augmenting its vigour under the influence of the stock—the supplying of a mentor as I call it—on the following year all the flowers of the grafts set fruit which developed completely. In general, even most of the simple hybrids fail to set fruit at their first blossoming, and even if fruit are formed their seeds sometimes fail to germinate; only in the years that follow do these shortcomings in development gradually disappear!


Fig. 37. Interspecific cross of Prunus Padus Maackii (left) X Prunus Cerasus (right). Hybrid in centre.

I could add scores of other examples to those cited above from my own observations and hundreds from the investigations of others, but I believe that these are a sufficient proof of the veracity of my conclusions on the matter. As to citing examples from and referring to the works of other authoritative investigators I consider it rather risky, since the main point of their achievements may be misrepresented.

Besides, it is generally not my custom to sprinkle my works with references to the investigations of others, if only because many of the postulations of authoritative persons are not very sound.

In general, I am opposed to all forms of pedantry and consider the propping up of my investigations by references to the works of others a form of needless cowardice in the face of criticism.