Transactions of the Horticultural Society 5: 292-296 (1824)
XXXIX. An Account of some Mule Plants.
By THOMAS ANDREW KNIGHT, Esq. F.R.S. &c. President.

Read May 6, 1823.

* Horticultural Transactions, Vol. iv. page 16.

THE excessive rarity of Mule Plants, in a perfectly wild state, (if in such they exist at all), and the facility with which they are, in many cases, obtained in the garden, seem to countenance the opinion, which is entertained by many botanists, that plants of different species do not readily breed with each other, till their natural habits have been broken and changed by the operation of culture through some successive generations. Vegetable mules are, however, never produced except under circumstances which rarely, if ever, occur in a perfectly natural state; for experiment has satisfied me, that not only the pollen of the alien species must be introduced at the proper period, but also, that the natural pollen must be kept away not only at that precise period, but generally, for several succeeding days afterwards: also, and even under the most favourable circumstances, I have never succeeded in obtaining mules, unless the plant or a considerable branch of a fruit tree, has been reduced to the necessity of nourishing mule offspring, or none. When the later blossoms on a fruit tree were suffered to remain, such branch either threw off the fruit which would have afforded mule plants, or the natural pollen was found to have been subsequently introduced by insects, or winds, and to have annihilated the operation of that obtained from the plant of another species. Not improbably some erroneous conclusions may also have been drawn, owing to varieties of permanent habits into which different species of plants have sported under the influence of different soils and climates in a perfectly natural state, having been mistaken for originally distinct species, for I perfectly agree with Mr. HERBERT,* in thinking, that the number of species of plants, which came immediately from the hand of nature, is probably much smaller than that now found in the catalogues of botanical writers: and it is also wholly impossible to distinguish such natural varieties from originally distinct species, by any peculiarities in their external character. In the present imperfect and limited state of our information, it is therefore, in many cases, difficult to decide whether plants are, or are not, mules; it being still questionable whether mere natural varieties, after they have, through successive generations, assumed very widely different forms and characters, are found to breed with each other as readily, as other varieties of the same species, of similar habits; and that real mule plants have in some instances, and under certain circumstances, produced offspring, (mules like themselves, I suspect), cannot, I believe, be questioned.

The principal object of the present communication is to describe two new kinds of mule plants, which have recently come within my observation. One of these presents the singularity of being, though certainly a mule, in some degree deserving the attention of the fruit-gardener; and the other affords me the means of pointing out a new species of fruit, in the Morello Cherry, to the improvement of which I wish particularly to invite the attention of the experimental gardener.

The results of many experiments upon, the different kinds of Strawberries, which are cultivated in our gardens, led me, some years ago, to conclude that we possess three distinct species of that genus; the Wood or Alpine, the Scarlet in many states of variation, and the Hautbois. I failed to obtain mule plants between the Alpine and the Scarlet, and Hautbois, which I inferred to be of distinct species, because they did not, under favourable circumstances, breed at all with each other. But I have subsequently seen, in the possession of my friend, Mr. WILLIAMS, of Pitmaston, mule plants obtained from the seeds both of the Scarlet, and Hautbois, and the pollen of the Alpine Strawberry. One of these, which sprang from the seed of the Hautbois, presents in its foliage and habit the character of its female parent, without any perceptible variation. It blossoms very freely, and its blossoms set well; but the growth of the fruit subsequently remains very nearly stationary during the whole period in which the Hautbois Strawberry grows, and ripens; after which it swells and acquires maturity. It is then rich and high flavoured, but of less size than the Hautbois, and without seeds. Mr. WILLIAMS, however, informed me, that he had once obtained a single seed, which afforded a mule plant in every respect similar to its parent. I have sent a few plants of each kind to our garden, and I believe the varieties will be thought to deserve culture by those who are admirers of the flavour of the Hautbois, and wish to prolong its season. The plants in my garden afford a second blossom in autumn.

Not entertaining any doubt of the specific identity of the Morello and common Cherry, I made experiments upon a large scale, confidently anticipating the production of some very valuable new varieties; and I had in consequence not less than twenty trees, which afforded blossoms in the last season. Buds of many of these had been inserted into the bearing branches of old Cherry trees, which were trained to walls of different aspects; and blossoms, which were all apparently well organized and perfect, were every where abundantly produced, but very nearly all proved abortive. From a south wall I obtained five Cherries from nearly as many thousand blossoms, and four of these did not contain seeds. One variety was very large, and nearly similar in colour to its male parent, the Elton Cherry; but its colour was somewhat deeper. Its flesh was white and melting, with very abundant juice; but containing only a small portion of saccharine matter. The others were worthless, and all the plants are, I believe, unquestionably mules.

As a species of fruit I consider the Morello Cherry to present very strong claims to the attention of the Horticulturist. The hardiness of its blossoms, which I have found to be alike patient of heat and cold; the large size of the fruit, with its abundant juice, and power of retaining its soundness and perfection long after it has become mature; and the exuberant produce of the tree in situations where the common Cherry succeeds but ill, render it, with all its present imperfections most valuable: and there appears to be no reasonable ground for doubt, but that richer and possibly larger varieties of it may be generated by proper culture through a few successive generations. Should the fruit become rich, a less exuberant produce must, however be expected; for sugar appears to be an article, the production of which requires a large expenditure of the vital juices of the tree.

We possess, I believe, in the Flemish and Kentish Cherry, two varieties of the same species with the Morello; and the Tousaint, and one or two others described by DUHAMEL in his Traités des Arbres Fruitiers, appear to belong to the same family. The Morello Cherry tree is obviously the "Cérisier très fertile" of this author.

I have seen the blossoms and fruit of the Morello Cherry tree bear, in the forcing-house, the temperature of seventy and even of eighty degrees, without any injurious or peculiar effects, except that the plumules of the seeds produced in such high temperature expanded with something very like blossoms upon the points. Small white leaves, in every respect similar to the petals of blossoms, were in many instances arranged as in a perfect blossom, which withered and died, whilst a bud upon the lower part of the stem vegetated, and the period of puberty in the plants did not subsequently appear to be at all accelerated by the operation of the high temperature in which the seeds had been ripened.

I do not offer plants of the Mule varieties above-mentioned of the Cherry, to the Society, because I feel quite confident of their being wholly useless.