Popular Science Monthly/Volume 84/February 1914/Some Abnormalities in Apple Variation
|SOME ABNORMALITIES IN APPLE VARIATION|
ASSISTANT HORTICULTURIST, WASHINGTON EXPERIMENT STATION
IF one were to enumerate the chief characteristics of the apple, its variability would doubtless come well toward the head of the list. A species which has been so long under domestication, which has been removed to so many localities where it is not native and subjected to conditions so different from those of its original habitat and which has such a complex ancestry as our modern apple, may well be expected to display a great variety of forms, and among the number some of such abnormal character and infrequent occurrence as to be reckoned as curiosities. It is not my intention in this article to discuss the variation of the apple in general, but only to jot down some notes concerning a few such freaks as have come to my attention during the past season.
First of all, I wish to consider some curiosities of apple coloration. It is pretty generally understood that, other conditions remaining the same, the color of an apple depends to a great extent upon the amount of sunlight to which the fruit has been exposed. The apples grown on the shady side of the tree are apt to be somewhat poorly colored, and the shady side of an apple itself is nearly always less highly colored than the side exposed to the direct rays of the sun. Every one is familiar with the nicely contrasting light spots which are common on our dark red apples where they have been shaded by some friendly leaf, and such spots, if not too extensive, are usually regarded as adding to the attractiveness of the fruit. This effect of sunlight and shadow upon the color of an apple is so well understood that it is often made use of in printing various designs upon the surface of the fruit. It is not often, however, that an effect of this kind is produced wholly without intention, yet that such a thing may happen is shown in the illustration. This specimen of nature's color photography represents a leaf with petiole, midrib and marginal teeth. The apple is of the Mcintosh variety and is one of a number which resulted from artificial cross pollination. In order to prevent the loss of the apples in case they should drop from the tree before picking time, they were enclosed early in the season in small sacks of mosquito netting. This particular specimen happened to so press against the interior of the sack that a leaf was held firmly against its side, and the nearly perfect print of the leaf was the result. The faint lines which appear in the dark portion of the surface are the prints of the threads forming the sack.
Though the effect of the sunlight upon the color of apples is so well known, the nature of the pigment formed and the changes which it may undergo are not well understood. It is apparent from chemical studies that iron is present in its composition and this has led to the belief in some quarters that a soil rich in iron compounds is a factor of prime importance for the production of highly colored fruit. As a matter of fact, iron is also necessary to the production of chlorophyll in the leaves,
Print of a Leaf upon the Surface of an Apple about One Half Natural Size.
and since the ash of the leaves contains a larger proportion of iron than that of the fruit, it seems fair to suppose that soils containing sufficient iron for proper chlorophyll development also have an abundance for the formation of pigment in the fruit.
Even more obscure are the causes which lead to the formation of pigment in certain varieties while it is absent or nearly so in others. While the color is no doubt greatly influenced by heredity, this fact takes us but a short step nearer the solution of the problem. It was formerly held that not only the color but also the form, size and flavor of an apple might be greatly influenced by the variety furnishing the pollen with which the blossom is fertilized. This theory is generally discredited at the present time, as well authenticated instances of such direct effect of the pollen are lacking in the case of the apple. The direct effect of pollen upon the color of the fruit is the rule in corn, as is well known, and it is not impossible that it might sometimes be manifest in the apple, though proof of that fact is lacking. At any rate, specimens are sometimes found which strongly suggest the direct influence of the pollen, and it has been my fortune to examine a half dozen such apples last season.
The first to come to my attention was grown in the Lake Chelan locality and appeared to be a small specimen of the Ben Davis type.
Particolored Apple Usually Ascribed to Bud Variation but Sometimes Regarded as Showing the Direct Effect of Pollen. About one half natural size.
It had been picked for some time when received and was badly shriveled. The larger part of the surface of the apple was greenish indistinctly striped with red. A segment, however, covering about two fifths of the surface was of a very dark color with scarcely a suggestion of striping. The lines of demarcation between these dissimilar portions of the surface were clearly and sharply marked, as shown in the illustration, and extended from stem to calyx. Upon cutting the apple, it was found that the dark portion of the surface just covered two of the carpels or divisions of the core. This portion contained three perfect seeds, while there were five seeds in the remainder of the core.
The second specimen of this kind was a Mcintosh apple from the experiment station orchard. In this case the larger portion of the surface was of the normal red color of the variety, but a segment of about one third of the surface was very light in color, faintly streaked with pink. The two sections of the surface were clearly marked off from each other, though the lines of demarcation were not quite so sharp and regular as in the specimen already described. The light-colored segment covered approximately two carpels of the core, although the open nature of the Mcintosh core made it more difficult to determine that point accurately in this apple than in the other. Each part of the core contained four seeds. Two of these were abnormal and will be described farther on in this paper.
A poorly colored apple of the Arkansas variety received from White Salmon had a brownish red band extending from stem to calyx, covering a fifth of the surface. On the opposite side of the apple was a similar streak, but only about an eighth of an inch wide. The dark portion included one carpel containing a single seed, while the remainder of the fruit contained but one good seed. A Winesap from the Yakima Valley had on the lighter-colored side a deep red stripe covering about an eighth of the surface and including one carpel. This portion of the core contained one seed while there were seven seeds in the remainder of the fruit. A Rome grown at Pullman had half of the surface of a nearly solid red color, the remainder being green with red splashes. The lighter portion contained three seeds and the dark part two. A second specimen of Rome from Pullman had a dark area covering two carpels, an area of moderately light color, also covering two carpels, and a still lighter portion covering one carpel which was seedless. The other portions contained three seeds each. The seeds from the differently colored portions of all these apples were saved separately and planted in the hope of obtaining some light in regard to the significance of such abnormalities.Turning from freaks of color, we will next consider some apples of abnormal structure. The doubling of fruits or the multiplication of parts is a variation of less common occurrence in the apple than in some other species. Certain varieties of the plum, for example, ordinarily produce a fairly large proportion of double fruits. These result from the presence of two complete pistils in a single blossom. Analogous cases of polycarpy occur in the apple, though less frequently than in the stone fruits. Apples having six carpels have come under my notice in the Grimes, Rome, Gano, Delicious, Chelan and Yellow Newtown varieties, and I have observed blossoms of the Golden Sweet having six pistils. The Yellow Newtown seems especially prone to this kind of variation and a number of specimens were found the past season containing six carpels, and in one instance a fruit with seven carpels was noted. The
Polycarpy in the Apple. Yellow Newtown apple, having seven carpels, Natural size.
latter specimen had six calyx lobes, though the multiplication of carpels seems to take place as a rule independently of the other floral organs. An extra sepal, or possibly a bract, below the calyx and on the side of the tube, is not so very uncommon on fruits otherwise normal in structure and persists in the fruit as a scale on the side of the apple, usually deforming it somewhat.
I have never observed the suppression of carpels in apples of the ordinary varieties, though it may occur in some of the so-called seedless or coreless forms. In case one or more of the pistils fail to receive pollen while the remainder are successfully pollinated, the corresponding carpels appear always to develop more or less with the growth of the fruit, though they remain empty.Double apples, though comparatively rare, have occasionally been described and two such fruits were found at Pullman last season, one in the orchard of the experiment station, the other in an orchard adjoining the college campus, while a third was received from Wenatchee.
Polycarpy in the Apple. Grimes' apple having six carpels. Slightly reduced.
The first, a specimen of the Ben Davis, consisted of two nearly independent apples united for only a short distance near the base. The stem, originally single, had been split nearly to the base, while that part of the fruit between the insertion of the stem and the point of union of the apples had been broken apart as the apples had developed at the base. The two portions, thus almost completely separated, had each developed into a nearly perfect apple, though of small size. The cores were perfect and the axes, which were nearly straight, extended in almost opposite directions.
The second specimen, a Jonathan, consisted of two apples of unequal size joined together throughout their entire length and marked off from each other only by a shallow groove. The single stem upon which the fruit was borne was split for a short distance as the two portions spread apart in growing. When cut open each section was found to have an independent core containing perfect seeds, although in the smaller part that portion of the core adjacent to the other part was undeveloped. The axes of the cores, moreover, were strongly curved away from each other. The specimen from Wenatchee was similar in all important respects to the one just described and further account of it will not be necessary. Both of the double apples grown at Pullman are shown whole and in section in the accompanying illustrations.Double apples of this kind originate in a manner quite different from the double plums mentioned above. The presence on the mature apple of two separate calyx cups indicates that these apples were produced by distinct blossoms borne at the end of a single flower stalk. In
the apples last described the receptacles of the blossoms must have been united nearly to the base of the sepals. In the first specimen, however, union appears to have taken place only at the base.
Polyembryony, or the multiplication of the embryos in the seed, is a condition normal in certain cases, notably in the citrus fruits and the Mango. Such seeds arise from ovules containing a single egg cell which gives rise to one of the embryos; the others originate as outgrowths, by a sort of budding process, from the inner wall of the embryo sack and of course reproduce the parent form as perfectly as other methods of bud propagation. True polyembryony must not be confused with the presence of more than one seed in an indehiscent fruit which normally contains a single seed. Such an occurrence is quite common in the peach and related fruits where the stone is really the endocarp or inner portion of the fruit and may enclose two seeds produced independently of each other, though the normal number is but one.The carpels of the apple may contain from one to four seeds each, though the most common number appears to be two. I am not aware that a polyembryonic apple seed has been reported up to this time; however, one of the seeds in the abnormally colored Mcintosh apple already described contained two embryos, while a second seed of the same character was found in another specimen of this variety. The manner of origin of these accessory embryos was not determined, though each embryo appeared to be surrounded by an independent inner seed coat. This fact would seem to indicate a method of production different from
that of the polyembryonic seeds already noted, perhaps by a sort of fasciation or doubling of the ovule and the production in it of two distinct egg cells.
The seeds of the Mcintosh apple appear, indeed, to abound in anomalous forms. Another seed in the same abnormally colored apple was remarkable for its small size, being perfectly formed, but of scarcely one fourth the length of an ordinary seed. Other seeds of this variety have been found in which the seed coat failed to develop and the embryo grew to full size without the usual brown covering, traces of which were found as a small patch at the hilum. An instance of this kind has also been noted in a seed of the Rhode Island Greening. In other cases the seed coats have split open as the seed developed, apparently as the result of the excessive production of endosperm which protruded as an irregular whitish mass containing the embryo. Though it is possible that such forms are the result of some unnatural state of nutrition in the seeds of this variety, nothing is known of the conditions which give rise to their production. Owing to the comparatively infrequent occurrence of abnormal forms, an investigation of their underlying causes is more difficult than the study of normal variation and progress is consequently slower.
- Since these notes were written a student has discovered specimens of the Rome and Gano varieties having four carpels and very rarely three carpels; also seven carpels in the latter variety.