On the direct or immediate action of the Male Element on the Mother Form.—Another remarkable class of facts must be here considered, because they have been supposed to account for some cases of bud-variation: I refer to the direct action of the male element, not in the ordinary way on the ovules, but on certain parts of the female plant, or in the case of animals on the subsequent progeny of the female by a second male. I may premise that with plants the ovarium and the coats of the ovules are obviously parts of the female, and it could not have been anticipated that they would be affected by the pollen of a foreign variety or species, although the development of the embryo, within the embryonic sack, within the ovule, within the ovarium, of course depends on the male element.
Even as long ago as 1729 it was observed[[928]] that white and blue varieties of the Pea, when planted near each other, mutually crossed, no doubt through the agency of bees, and in the autumn blue and white peas were found within the same pods. Wiegmann made an exactly similar observation in the present century. The same result has followed several times when a variety with peas of one colour has been artificially crossed by a differently-coloured variety.[[929]] These statements led Gärtner, who was highly sceptical on the subject, carefully to try a long series of experiments: he selected the most constant varieties, and the result conclusively showed that the colour of the skin of the pea is modified when pollen of a differently coloured variety is used. This conclusion has since been confirmed by experiments made by the Rev. J. M. Berkeley.[[930]]
Mr. Laxton of Stamford, whilst making experiments on peas for the express purpose of ascertaining the influence of foreign pollen on the mother-plant, has recently[[931]] observed an important additional fact. He fertilised the Tall Sugar pea, which bears very thin green pods, becoming brownish-white when dry, with pollen of the Purple-podded pea, which, as its name expresses, has dark-purple pods with very thick skin, becoming pale reddish-purple when dry. Mr. Laxton has cultivated the tall sugar-pea during twenty years, and has never seen or heard of it producing a purple pod; nevertheless, a flower fertilised by pollen from the purple-pod yielded a pod clouded with purplish-red, which Mr. Laxton kindly gave to me. A space of about two inches in length towards the extremity of the pod, and a smaller space near the stalk, were thus coloured. On comparing the colour with that of the purple-pod, both pods having been first dried and then soaked in water, it was found to be identically the same; and in both the colour was confined to the cells lying immediately beneath the outer skin of the pod. The valves of the crossed pod were also decidedly thicker and stronger than those of the pods of the mother-plant, but this may have been an accidental circumstance, for I know not how far their thickness in the Tall Sugar-pea is a variable character.
The peas of the Tall Sugar-pea, when dry, are pale greenish-brown, thickly covered with dots of dark purple so minute as to be visible only through a lens, and Mr. Laxton has never seen or heard of this variety producing a purple pea; but in the crossed pod one of the peas was of a uniform beautiful violet-purple tint, and a second was irregularly clouded with pale purple. The colour lies in the outer of the two coats which surround the pea. As the peas of the purple-podded variety when dry are of a pale greenish-buff, it would at first appear that this remarkable change of colour in the peas in the crossed pod could not have been caused by the direct action of the pollen of the purple-pod: but when we bear in mind that this latter variety has purple flowers, purple marks on its stipules, and purple pods; and that the Tall sugar-pea likewise has purple flowers and stipules, and microscopically minute purple dots on the peas, we can hardly doubt that the tendency to the production of purple in both parents has in combination modified the colour of the peas in the crossed pod. After having examined these specimens, I crossed the same two varieties, and the peas in one pod, but not the pods themselves, were clouded and tinted with purplish-red in a much more conspicuous manner than the peas in the uncrossed pods produced at the same time by the same plants. I may notice as a caution that Mr. Laxton sent me various other crossed peas slightly, or even greatly, modified in colour; but the change in these cases was due, as had been suspected by Mr. Laxton, to the altered colour of the cotyledons, seen through the transparent coats of the peas; and as the cotyledons are parts of the embryo, these cases are not in any way remarkable.
Turning now to the genus Matthiola. The pollen of one kind of stock sometimes affects the colour of the seeds of another kind, used as the mother-plant. I give the following case the more readily, as Gärtner doubted similar statements with respect to the stock previously made by other observers. A well-known horticulturist, Major Trevor Clarke, informs me[[932]] that the seeds of the large red-flowered biennial stock (Matthiola annua; Cocardeau of the French) are light brown, and those of the purple branching Queen stock (M. incana) are violet-black; and he found that, when flowers of the red stock were fertilised by pollen from the purple stock, they yielded about fifty per cent. of black seeds. He sent me four pods from a red-flowered plant, two of which had been fertilised by their own pollen, and they included pale brown seed; and two which had been crossed by pollen from the purple kind, and they included seeds all deeply tinged with black. These latter seeds yielded purple-flowered plants like their father; whilst the pale brown seeds yielded normal red-flowered plants; and Major Clarke, by sowing similar seeds, has observed on a greater scale the same result. The evidence in this case of the direct action of the pollen of one species on the colour of the seeds of another species appears to me conclusive.
In the foregoing cases, with the exception of that of the purple-podded pea, the coats of the seeds alone have been affected in colour. We shall now see that the ovarium itself, whether forming a large fleshy fruit or a mere thin envelope, may be modified by foreign pollen, in colour, flavour, texture, size, and shape.
The most remarkable instance, because carefully recorded by highly competent authorities, is one of which I have seen an account in a letter written, in 1867, by M. Naudin to Dr. Hooker. M. Naudin states that he has seen fruit growing on Chamærops humilis, which had been fertilised by M. Denis with pollen from the Phœnix or date-palm. The fruit or drupe thus produced was twice as large as, and more elongated than, that proper to the Chamærops; so that it was intermediate in these respects, as well as in texture, between the fruit of the two parents. These hybridised seeds germinated, and produced young plants likewise intermediate in character. This case is the more remarkable as the Chamærops and Phœnix belong not only to distinct genera, but in the estimation of some botanists to distinct sections of the family.
Gallesio[[933]] fertilised the flowers of an orange with pollen from the lemon; and one fruit thus produced bore a longitudinal stripe of peel having the colour, flavour, and other characters of the lemon. Mr. Anderson[[934]] fertilised a green-fleshed melon with pollen from a scarlet-fleshed kind; in two of the fruits "a sensible change was perceptible; and four other fruits were somewhat altered both internally and externally." The seeds of the two first-mentioned fruits produced plants partaking of the good properties of both parents. In the United States, where Cucurbitaceæ are largely cultivated, it is the popular belief[[935]] that the fruit is thus directly affected by foreign pollen; and I have received a similar statement with respect to the cucumber in England. It is known that grapes have been thus affected in colour, size, and shape: in France a pale-coloured grape had its juice tinted by the pollen of the dark-coloured Teinturier; in Germany a variety bore berries which were affected by the pollen of two adjoining kinds; some of the berries being only partially affected or mottled.[[936]] As long ago as 1751[[937]] it was observed that, when differently coloured varieties of maize grow near each other, they mutually affect each other's seeds, and this is now a popular belief in the United States. Dr. Savi[[938]] tried the experiment with care: he sowed yellow and black-seeded maize together, and on the same ear some of the seeds were yellow, some black, and some mottled,[[939]] the differently coloured seeds being arranged in rows or irregularly. Mr. Sabine states[[940]] that he has seen the form of the nearly globular seed-capsule of Amaryllis vittata altered by the application of the pollen of another species, of which the capsule has gibbous angles. Mr. J. Anderson Henry[[941]] crossed Rhododendron Dalhousiæ with the pollen of R. Nuttallii, which is one of the largest-flowered and noblest species of the genus. The largest pod produced by the former species, when fertilised with its own pollen, measured 1-2/8 inch in length and 1½ in girth; whilst three of the pods which had been fertilised by pollen of R. Nuttallii measured 1⅝ inch in length and no less than 2 inches in girth. Here we see the effect of foreign pollen apparently confined to increasing the size of the ovarium; but we must be cautious in assuming, as the following case shows, that in this instance size has been directly transferred from the male parent to the capsule of the female plant. Mr. Henry fertilised Arabis blepharophylla with pollen of A. Soyeri, and the pods thus produced, of which he was so kind as to send me detailed measurements and sketches, were much larger in all their dimensions than those naturally produced by either the male or female parent-species. In a future chapter we shall see that the organs of vegetation in hybrid plants, independently of the character of either parent, are sometimes developed to a monstrous size; and the increased size of the pods in the foregoing cases may be an analogous fact.
No case of the direct action of the pollen of one variety on another is better authenticated or more remarkable than that of the common apple. The fruit here consists of the lower part of the calyx and of the upper part of the flower-peduncle[[942]] in a metamorphosed condition, so that the effect of the foreign pollen has extended even beyond the limits of the ovarium. Cases of apples thus affected were recorded by Bradley in the early part of the last century; and other cases are given in old volumes of the Philosophical Transactions;[[943]] in one of these a Russeting apple and an adjoining kind mutually affected each other's fruit; and in another case a smooth apple affected a rough-coated kind. Another instance has been given[[944]] of two very different apple-trees growing close to each other, which bore fruit resembling each other, but only on the adjoining branches. It is, however, almost superfluous to adduce these or other cases, after that of the St. Valery apple, which, from the abortion of the stamens, does not produce pollen, but, being annually fertilised by the girls of the neighbourhood with pollen of many kinds, bears fruit, "differing from each other in size, flavour, and colour, but resembling in character the hermaphrodite kinds by which they have been fertilised."[[945]]
I have now shown, on the authority of several excellent observers, in the case of plants belonging to widely different orders, that the pollen of one species or variety, when applied to a distinct form, occasionally causes the coats of the seeds and the ovarium or fruit, including even in one instance the calyx and upper part of the peduncle of the mother-plant, to become modified. Sometimes the whole of the ovarium or all the seeds are thus affected; sometimes only a certain number of the seeds, as in the case of the pea, or only a part of the ovarium, as with the striped orange, mottled grapes and maize, are thus affected. It must not be supposed that any direct or immediate effect invariably follows the use of foreign pollen: this is far from being the case; nor is it known on what conditions the result depends. Mr. Knight[[946]] expressly states that he has never seen
the fruit thus affected, though he has crossed thousands of apple and other fruit-trees. There is not the least reason to believe that a branch which has borne seed or fruit directly modified by foreign pollen is itself affected, so as subsequently to produce modified buds: such an occurrence, from the temporary connection of the flower with the stem, would be hardly possible. Hence but very few, if any, of the cases of sudden modifications in the fruit of trees, given in the early part of this chapter, can be accounted for by the action of foreign pollen; for such modified fruits have commonly been afterwards propagated by budding or grafting. It is also obvious that changes of colour in the flower which necessarily supervene long before it is ready for fertilisation, and changes in the shape or colour of the leaves, can have no relation to the action of foreign pollen: all such cases must be attributed to simple bud-variation.
The proofs of the action of foreign pollen on the mother-plant have been given in considerable detail, because this action, as we shall see in a future chapter, is of the highest theoretical importance, and because it is in itself a remarkable and apparently anomalous circumstance. That it is remarkable under a physiological point of view is clear, for the male element not only affects, in accordance with its proper function, the germ, but the surrounding tissues of the mother-plant. That the action is anomalous in appearance is true, but hardly so in reality, for apparently it plays the same part in the ordinary fertilisation of many flowers. Gärtner has shown,[[947]] by gradually increasing the number of pollen-grains until he succeeded in fertilising a Malva, that many grains are expended in the development, or, as he expresses it, in the satiation, of the pistil and ovarium. Again, when one plant is fertilised by a widely distinct species, it often happens that the ovarium is fully and quickly developed without any seeds being formed, or the coats of the seeds are developed without an embryo being formed within. Dr. Hildebrand also has lately shown in a valuable paper[[948]] that, with several Orchideæ, the action of the plant's own
pollen is necessary for the development of the ovarium, and that this development takes place not only long before the pollen-tubes have reached the ovules, but even before the placentæ and ovules have been formed; so that with these orchids the pollen apparently acts directly on the ovarium. On the other hand, we must not overrate the efficacy of pollen in this respect; for in the case of hybridised plants it might be argued that an embryo had been formed and had affected the surrounding tissues of the mother-plant before it perished at a very early age. Again, it is well known that with many plants the ovarium may be fully developed, though pollen be wholly excluded. And lastly, Mr. Smith, the late Curator at Kew (as I hear through Dr. Hooker), observed the singular fact with an orchid, the Bonatea speciosa, the development of the ovarium could be effected by mechanical irritation of the stigma. Nevertheless, from the number of the pollen-grains expended "in the satiation of the ovarium and pistil,"—from the generality of the formation of the ovarium and seed-coats in sterile hybridised plants,—and from Dr. Hildebrand's observations on orchids, we may admit that in most cases the swelling of the ovarium, and the formation of the seed-coats, are at least aided, if not wholly caused, by the direct action of the pollen, independently of the intervention of the fertilised germ. Therefore, in the previously-given cases we have only to add to our belief in the power of the plant's own pollen on the development of the ovarium and seed-coats, its further power, when applied to a distinct species or variety, of influencing the shape, size, colour, texture, &c., of these same parts.
Turning now to the animal kingdom. If we could imagine the same flower to yield seeds during successive years, then it would not be very surprising that a flower of which the ovarium had been modified by foreign pollen should next year produce, when self-fertilised, offspring modified by the previous male influence. Closely analogous cases have actually occurred with animals. In the case often quoted from Lord Morton,[[949]] a nearly purely-bred, Arabian, chesnut mare bore a hybrid to a quagga; she was subsequently sent to Sir Gore Ouseley, and produced