Journal of Heredity 13: 323-328 (July 1922)

Bureau of Plant Industry, U. S. Department of Agriculture

DATA of special value in the study of diversity may be drawn from plant life or from such lower animals as worms, millipeds, or centipeds, with the body showing numerous repetitions of the same organ. Dozens or hundreds of joints of a metamerous plant or animal body may follow each other as exact biological equivalents, and such series may be considered as the closest approach to uniformity in the processes of reproduction. Since the metamers are products of segmentation or vegetative growth, the problem of interpretation is greatly simplified. Many physiological and environmental complications attend sexual reproduction, with endless possibilities of confusion regarding the heredity of sexually produced individuals. The individuality of the vegetative internodes is of a lower order, but lends itself better to a determination of the question of diversity.

That internode members of the same plant are formed under the same conditions and are of the same heredity, is hardly to be questioned. At least, it will be admitted that the conditions of internode development are more alike in the same plant than can be asserted for different plants. Of course, the vegetative internodes may be specialized in various ways and degrees, as are the internode components of the flowers and fruits, so that numerous types of internodes may be distinguished among the internode members of a plant, but hundreds or thousands of examples of the same type may be available for comparison in a tree, shrub, or large plant.

Biological Duplication

Not only do successive internodes or leaves of the same vegetative shoot represent biologically the same thing, but comparison of the two halves of the same leaf or other symmetrical parts of the same internode individual gives even a sharper focus upon the question whether biological duplication or manifolding of the same structure is a process of mechanical exactness or may follow many alternative courses, with a resulting diversity of detail.

Whether we think of a transmitted "character" as a general "determiner" of leaf‑form for the entire plant, or as restricted to a particular feature, or to an individual leaf, exact determination is hardly to be claimed when definite differences are shown by corresponding parts of the same leaf. (See figures 19 to 22.)

The meaning of such facts in relation to heredity depends, of course on the assumptions that are made regarding the nature and number of the transmitted determiners or "genes." In proportion as fewer determiners are assumed for the transmission of the characters a wider range of diversity must be admitted in the expression or development of the characters, to account for differences among the internode individuals of the same plant. The course of internode development is not simple and direct, but divides and ramifies into the many alternative paths that are shown in the formation of the different types of internodes, as well as in the diversity of form that appears among the internode individuals of the same type. Even in the same leaf, diversity may be shown, when equivalent parts are compared.

The different types of internodes on the same plant may be as different as species, genera or families, or may be considered as analogous to the sexes, castes or other definitely diverse forms that become established in many species. But in addition to these larger differences the internode individuals of the same type show a general diversity corresponding to that of individual plants or animals which are members of the same species. Thus the most direct indications of biological facts do not support the idea of definite paths of development, or of paths that are followed exactly. The inference is that diversity, rather than uniformity, should be considered as the normal product or manifestation of heredity. The different types of internodes of the same plant were considered by Goethe as an important evidence of evolution, and a like significance may be claimed for internode diversity in relation to heredity.

Having seen that the expression of characters is frequently and indefinitely varied in the development of the internode individuals, it seems not unreasonable to expect a similar variability in sexually produced individuals. And to consider such diversity as a result of normal reproductive processes. Goethe described the growth of internodes, one from another, as "successive reproduction," in contrast with "simultaneous reproduction'' by sexual processes.

Notwithstanding the great advances that have been made in recent years in the cytological study of transmission and the mechanical explanation of the inheritance of Mendelian differences, there still is no conception of the nature of the characters, as showing how they are represented in transmission, or how they are brought into expression. Hence it is possible as yet to think definitely of characters only as they are brought into expression, through the development of the plant or animal individual. The results that are produced are the only basis of judgment regarding the nature of the reproductive processes, the transmission and expression of the characters.

Diversity of leaf-forms in Pistacia

Diversity of leaf-forms in Pistacia
FIGURE 19. Leaves of the pistachio tree (Pistachia vera) at Sacaton, Arizona, Octoer, 1922, all from the same tree as Figure 20, the three large leaves from the same twig. The typical leaf-form is shown at the lower right-hand corner, a leaf with two pairs of lateral pinnae which are narrowed abruptly to the point of attachment while the terminal pinna has a gradually narrowed or decurrent base. Leaves with only one pair of lateral pinnae oftwn show a more abrupt narrowing of the base of the terminal pinna which in such cases probably indicates a complete fusion of the upper pair of lateral pinnae with the terminal pinna. Different stages of partial fusion are shown, as in Figure 20. FIGURE 20. Leaves from the same tree as in Figure 19, showing intermediate stages between leaves with one pair and two pairs of lateral pinnae. The second pair of lateral pinnae often is fused more or less completely with the terminal pinna. The lower left-hand leaf shows partial fusion on one side and complete fusion on the other, as indicated by the very abrupt base, which is a character of the lateral innae. In the upper left-hand leaf the decurrent base character is shown in the lateral pinnae, one of which is separate and the other completely fused with the terminal pinna. Such fluctuating differences in leaf-form are of interest as showing that the processes of heredity are not exact, but result in normal diversity.

Diversity of leaf-forms in agrimonia

Diversity of leaf-forms in agrimonia
FIGURE 21. Leaf of Agrimonia growing in partial shade at Lanham, Maryland, showing diversity among the pinnae, with many gradations in size,, and irregular alteration of arrangement. Groups of one to four of the small pinnae alternate with the large pinnae. The larger of the small pinnae are generally at the intermediate positions, between the bases of the large pinnae, but these relations are not regular, nor are the shapes or the marginal notches of the pinnae. FIGURE 22. For comparison with Figure 21 to show the nature of the differences that commonly appear among leaves of the same plant. The rachis of this leaf is longer and the pinnae more slender and widely spaced, also differently graded in size, with some of the secondary pinnae relatively larger, especially on the left side of the leaf. Also the upper, left-hand pinna is decurrent at base, possibly as a result of being united with a small pinna.

Cook bibliography

Evolution of Plant Structures