THE ORIGIN of each new seed-bearing plant traces back to the time of flowering. The end product of flowering is the seed. The thread of life that persists through the process of seed formation long has captivated the interest of man. If we wish to trace the processes that result in the transmission of life, we must start by examining the basic structural units, the cells of the parent plant.

The contents of cells are complex. Young cells of plants are filled with protoplasm, the physical basis of life. Its main components are the plastids, the nucleus, and the cytoplasm, a clear semifluid matrix, which contains the plastids, the nucleus, and many smaller bodies.

Plastids in green plants contain the chlorophylls, which give the plants their green color and enable them to carry on photosynthesis. Some plastids contain other pigments. Others carry no coloring matter but may contain starch or fat.

The nucleus is regarded as the specific center of many cell activities. The nucleus contains the chromosomes, which carry most of the hereditary materials of the plant.

The development of a number of scientific instruments made it possible for us to study in detail the contents of cells.

The electron microscope enables us to see particles that are only one five-hundredth to one one-thousandth as large as the smallest particles that can be resolved with the standard light microscope.

The phase contrast microscope permits examination of unstained living cells by converting slight differences in the indices of refraction of the cell parts to visible differences.

The ultraviolet microscope is immensely useful because of the selective absorption of ultraviolet light by nucleic acids.

The spectrometer permits detailed studies of the chemical composition through differential absorption by cell components of the different wavelengths of light.

With high-speed centrifuges one can separate the many different types of cellular particles.

Such detailed studies disclose that many bodies, much smaller than the plastids, exist in the protoplasm of the cells.

One group of such bodies are granular or rod shaped, are known as mitochondria, and are believed to give rise to plastids and to be involved in the metabolism of the cell.

Certain of the plastids and smaller bodies are transmitted from one generation of plants to the next through the sex cells and transmit a few characters from the mother plant to the offspring. We do not yet know the specific bodies that are involved.

The plant cell, as it matures, develops one or more cavities, the vacuoles within the cytoplasm. Vacuoles are filled with a water solution of sugars, salts, acids, and other substances. In large cells the cytoplasm eventually becomes a saclike layer surrounding a large vacuole.

One class of plant pigments, called the anthocyanins, at times occurs in solution in the vacuole. The red color of autumn leaves, for instance, is associated with pigments in the vacuole.

The chromosomes contain the factors, or genes, that govern development of most plant structures and traits. Chromosomes are the major means by which germ plasm, the vital stuff in the germ cells, is passed on from one generation to the next.

In a mature cell that is not dividing, the chromosomes are long, thin, fibrous threadlike bodies. Because they are so long and intertwined, we have not yet been able to examine a single whole chromosome.

Actually, we may say a chromosome is a bundle of fibrils, or threads. The number of the threads, or fibrils, seems to differ in different organisms. The chromosomes of corn may have just a few. Those of certain lilies may comprise eight fibrils.

Other organisms, in which chromosomes were tagged with radioactive substances or studied with the electron microscope, may have chromosomes of 32 to 64 units and possibly even 128 fibrils.

Studies of the chemistry of chromosomes indicate that they are comprised of complex organic molecules, including protein and ribose and deoxyribonucleic acids.

Irregularities in thickness and density occur along the length of a chromosome. The thickened parts, which resemble knots in a string, are called chromomeres. Some geneticists believe that chromomeres are accumulations of nucleic acids. Others believe they are expressions of different patterns of coiling along the chromosome thread.

No one has yet seen an individual gene. We can only speculate upon its nature and composition. Geneticists believe that genes, like chromosomes, consist of complex organic molecules, probably composed mainly of deoxyribonucleic acid, and that genes may differ from each other according to their molecular construction.

We know from genetic studies, however, that genes occur in linear order in the chromosome. This order is maintained through countless generations unless the chromosome is broken.

In corn, for instance, a gene locus, which influences the color of the endosperm, is located on a specific chromosome, known as the No. 6 chromosome, near a locus that controls the color of the plant. Scientists measure the distance between genes in terms of crossover units. These two genes are approximately 28 units apart. This distance remains constant generation after generation.

We know also that genes are stable, although occasionally they change, or mutate, to another form, as evidenced by the change in the character they influence.

We know that certain agencies, such as irradiation by X-rays, gamma rays, or ultraviolet rays, can increase the rate of mutation.

We know that certain genes mutate more frequently than others. Under normal conditions, however, most genes would not be expected to mutate oftener than once in hundreds of thousands or even millions of cell generations.

Genes reproduce themselves. Chromosomes duplicate themselves longitudinally. On the basis of genetic evidence, the genes must also be reproduced in kind.

Several times we have said a character, or trait, is caused or influenced by a gene. How, one asks, can a gene located on a chromosome in corn govern whether the color of the kernel will be red or yellow?

Here is another gap in our knowledge.

It is presumed that a gene of certain construction is responsible for the production of a specific enzyme within the protoplasm. The enzymes influence the activities of the cells and thereby determine the final expression of the character in question. We have only fragmentary evidence as to the way in which this is done.

INNUMERABLE cell divisions take place during the growth of a plant.

The dividing of vegetative cells is mitosis. The process is an orderly one—the individuality and stability of the number of chromosomes and the number of genes are maintained through the many cell cycles.

Let us examine this process.

During the early phases of nuclear division we can recognize the long, threadlike chromosomes, which contract and thicken to a degree that enables us to identify individual chromosomes. The contraction is actually a coiling of the chromosome, and the final form is somewhat like a coiled spring. When they are fully contracted, the coils may be closed so tightly that the chromosome appears to be a cylinder.

During mitosis, at some phase that has not been determined with absolute certainty, each chromosome becomes longitudinally visibly doubled and then consists of two chromatids, which are closely twisted around each other.

The physical doubling, or reduplication, of each chromosome provides that succeeding cells carry the exact genetic complement as the mother cell. As cell division proceeds, each of the chromatids develops into an individual chromosome in each of the two new cells that are formed.

Each of the chromatids consists of two further subdivisions, known as chromonemata. The two chromonemata comprising a chromatid are thought to be wound about each other very tightly, like twisted strands of a rope.

The chromonemata are the forerunners of chromatids of the next cell division and in time become the individual chromosomes in the four succeeding granddaughter cells.

When one projects this manner of regeneration of new chromosomes, it is not surprising to learn that the chromonemata in turn consist of further subdivisions and already carry the prototype of chromosomes for a number of cell generations in the future.

After the chromosomes contract, they migrate toward the center of the cell. A spindle-shaped figure of fibers forms in the cell. The membrane that encloses the nucleus had previously disappeared. The chromosomes become arranged in a central plane, which is perpendicular to the axis of the spindle.

The two chromatids that comprise each chromosome separate from each other and thus give rise to daughter chromosomes. The daughter chromosomes separate and move toward opposite ends of the cell.

The movement of the two chromatids is initiated at a constricted portion of the chromosome, known as the centromere, or spindle fiber attachment. The latter name is given this region because through the microscope it appears that a spindle fiber from one of the poles becomes attached at the restricted region of one chromatid, whereas a spindle fiber from the other pole similarly is attached to the sister chromatid.

The forces that cause the daughter chromosomes to migrate from each other and toward the poles are not fully understood. The daughter chromatids may be drawn toward the poles by the spindle fiber, or the daughter chromosomes may repel one another in the spindle fiber attachment region. In fact, it is not known if the spindle fibers are really fibers at all. They may be mere protoplasm arrangements, which demark lines of force that have developed in the nucleus.

Suffice it to say that the daughter chromosomes first separate in the centromere region and, as these regions move apart, the chromosomes uncoil until they are completely separated. Thereupon they migrate to the opposite poles of the cell.

Since this process takes place in each of the 20 chromosomes of corn, it follows that 20 daughter chromosomes migrate to each of the poles. A cell wall is formed between the separated groups of chromosomes, new nuclear membranes are formed, and the result is two daughter cells, each of which carries the same chromosome complement as the mother cell.

THE PROCESS of cell division may take only a few minutes or as much as many hours.

The significance of cell division as related to heredity may be summed up, first, as the exact duplication of each chromosome and the genes which it carries and, second, the mechanism that provides each of the daughter cells with the same chromosome complement as the parent cell.

Each vegetative cell of the plant therefore has the same complement of chromosomes and heredity potentials as the initial cell from which the plant arose.

SINCE THE complement of chromosomes and the genetic complement of all vegetative cells of the plant are alike, one may question why all cells of the plant are not identical in appearance when mature.

For instance, some cells develop into epidermis, the surface cells of plants. Other cells develop into the cells of the xylem and the phloem, through which materials are transported in the plant. Others become growth regions, such as cambium, which occurs under the bark of trees. Still others give rise to the egg or pollen grain, which are involved in sexual reproduction.

Differentiation of cells into the various tissues largely depends on the inheritance of tissue pattern, a subject that is not understood at present. The differentiation of cells, moreover, may be influenced by the neighboring cells or by the position in the plant in which the cells are located.

Regardless of the stimulus, we know that cells differentiate and fit a pattern of special uses and that such differentiation is controlled at least partly by the genes of the chromosomes carried in the cells.

ONE TYPE of cell differentiation that is particularly pertinent in a study of seeds is the production of sex cells in the flowers.

During the growth of a plant, certain groups of rapidly dividing cells, the floral primordia, form the flower structures. Flowers are produced on the tip of a stem, either of the main stem, as in the case of the corn tassel, or on the tip of a side branch, like an ear of corn.

The female flower cluster of the corn plant is differentiated from a group of meristematic cells in the axil of a leaf. This flower cluster eventually develops into an ear of corn. Flowers bearing the male portion of germ plasm are located at the top of the corn plant, and the inflorescence, or flower cluster, is known as the tassel.

Anthers, the organs that later shed pollen grains, are formed during the development of the male flowers in the tassel.

The cells of young anthers when seen under the microscope seem very much alike. As an anther enlarges, however, certain cells in each of the four lobes of the anther enlarge and become different from cells of the surrounding tissues. Each of the enlarged cells develops into a pollen mother cell, which divides twice to form four daughter cells—the microspores, which later develop into pollen grains.

When the anther is mature, it splits open. Then the pollen grains are released.

In the female flower cluster, which becomes the ear of corn, the process of cell division is somewhat different. The form of the ear and very young kernels can be recognized at a very early stage. The forerunner of the corn kernel is the ovary, which will become the kernel proper, and the stigma and style, which in combination constitute the silk.

The ovary contains an ovule, which fills the entire space in the ovary. One cell near the tip enlarges greatly as the ovule enlarges. This cell is the megaspore mother cell. It undergoes two divisions, providing four megaspores. Not all four of these megaspores are functional. Three of them disintegrate. One, usually the cell nearest the base of the ovule, continues to develop.

The cell divisions that produce the microspores and megaspores warrant special attention. The nuclei in these spores contain only half as many chromosomes as the spore mother cells. This reduction in number of chromosomes is brought about by two distinctive cell divisions, a process known as meiosis or reduction division.

When we discussed mitosis, or vegetative cell division, in corn, we stated that each nucleus contains 20 chromosomes, and after a mitotic division each daughter nucleus contains 20 chromosomes.

A study of the chromosomes in the contracted stage in a vegetative cell of corn shows that they constitute 10 pairs of chromosomes. The two chromosomes of each pair are structurally identical and genetically similar. They are said to be the homologous chromosomes.

The early stages of meiosis in the spore mother cell resemble vegetative cell division in that the chromosomes contract and thicken until they may be observed as distinct individual bodies.

At that point, however, a special phenomenon occurs. The homologous members of a pair of chromosomes become attracted to each other and become closely associated throughout their length. Coiling of the chromosome pairs results in a further shortening and thickening. The pairs become oriented in a plane at right angles to the axis between the poles at opposite ends of the cell. The members of each pair of chromosomes begin to separate at the centromere region, and the members migrate to opposite ends of the cell after uncoiling.

This process, which occurs in each of the 10 pairs of chromosomes, results in a complement of 10 single chromosomes at opposite sides of the mother cell. This represents half the number of chromosomes in a vegetative cell—hence the term "reduction division" applies to this part of meiosis.

Usually without pause, the chromosomes at each end of the cell undergo a second division. Each of the chromosomes, in which two chromatids are now evident, becomes oriented in a plane at right angles to their previous plane of orientation. Spindle fiber poles develop on opposite sides of the cell, with their axis perpendicular to the new plane of orientation.

The two chromatids comprising each chromosome thereupon separate from each other and migrate to the opposite poles. This cell division, which may be said to be equational, should be recognized as structurally identical to that occurring in vegetative cells, except that only half the number of chromosomes are involved. The two divisions of meiosis in the spore mother cell produce four spores, each containing one member of the homologous pairs of chromosomes that were present in vegetative cells.

The meiotic mechanism, followed by fertilization, assures constancy in the number of chromosomes in successive generations.

Following meiosis in the female flower, the nucleus of the surviving megaspore undergoes three successive mitotic divisions. An embryo sac with eight nuclei is produced. The nuclei form a pattern that is characteristic of the particular species.

Three nuclei are often located near the tip of the embryo sac. One nucleus becomes the egg cell. The other two, the synergids, are commonly nonfunctional.

Two nuclei, the polar nuclei, migrate to the center of the embryo sac.

The remaining three nuclei, the antipodals, are located at the base end of the embryo sac. In the corn plant, the antipodals undergo limited mitosis, but they have no known function and soon "disappear."

The nuclei that are concerned with fertilization are the egg nucleus and the polar nuclei.

At this stage of development of the embryo, the silk of the young corn kernel begins to elongate rapidly and soon protrudes from the tip of the husk.

When grains of corn pollen, which mostly are carried by wind, land on the branched silk, they germinate immediately and produce pollen tubes. The tubes penetrate the cells of the silk and grow down the silk through sheath cells that surround the vascular tissues. Before shedding, the micro-spore nucleus of the pollen grain of corn undergoes mitotic division and produces a tube nucleus and a generative nucleus. The latter divides and produces the two sperms, the male nuclei that take part in the fertilization process. The tube nucleus seems to have a function directly concerned with the growth of the pollen tube down the silk and into the ovule. It remains near the tip of the growing pollen tube.

The pollen tube can grow rapidly down the silk of the ear. The distance from the tip of the silk to the ovule may be as much as 12 inches. The pollen tube may reach it in about 24 hours.

After reaching the base of the silk, the pollen tube grows through additional sheath cells until it reaches the ovule cavity. It twists and turns through this cavity until it reaches the opening, or micropyle, left by incomplete closing of the integuments that cover the ovule.

The pollen tube thereupon enters the embryo sac, the tip enlarges greatly, and the tube membrane disintegrates. Then the two sperm nuclei are released.

One male gamete fuses with the nucleus of the egg, thereby producing a zygote. This is fertilization, and the zygote is the first cell of the embryo.

This one cell, in which all potentialities of structural and functional development are present, may be said to be a living plant—a corn plant.

The other male gamete fuses with the two polar nuclei, producing a triple-fusion nucleus that carries three homologous chromosomes of each of the ten distinct chromosomes in corn.

Further divisions of this nucleus produce the endosperm, which is a food storage structure in the seed of corn.

The fusion of one male gamete with the egg and the other male gamete with the polar nuclei is referred to as double fertilization. It occurs in most flowering plants.

Every cell of the embryo contains the same genetic constitution. Minor differences in structure and striking differences in cell activity and orientation soon become evident, however.

The symmetrical profile of the embryo persists only until lateral organs are initiated in orderly sequence and position.

Indications of unsymmetrical growth can be detected 4 to 5 days after pollination in a zone of cells on the upper and outward region of the embryo. These cells are less vacuolate, more deeply stainable, and undergo more rapid cell division than cells in other regions of the embryo.

This activity foreshadows the subsequent rapid elongation of the embryo and the initiation of lateral organs. The basal portion of the embryo, the suspensor, elongates considerably in some varieties of corn. The suspensor eventually becomes undercut and fractured by the scutellum.

The first lateral organ, the coleoptile, arises on the outward (anterior) surface of the embryo. The coleoptile eventually becomes the hollow cone that emerges from the soil during germination and contains the plumule, or aerial portion, of the plant.

The apex of the future stem, on which the tassel develops in due time, can be identified in 10 or 12 days. The first foliage leaf is evident in 12 days. The radicle, or first root of the embryo, usually is well defined at 10 to 12 days, deep in the tissues of the embryo.

At about this age, cell division is accelerated over an extensive area on the inward (posterior) surface of the embryo. This process produces the large, flat, shield-shaped scutellum, which has been interpreted to be the cotyledon of the grass embryo. Two meristematic ridges that arise on the anterior side of the scutellum eventually grow around and encase the plumule-radicle axis of the embryo.

The foregoing observations show that the first member of each organ category, stem, leaf, and root, is initiated soon after fertilization, and the development of the embryonic corn plant is well underway.

Additional foliage leaves are initiated at intervals of 4 or 5 days. In 30 to 35 days, the full complement of five embryonic foliage leaves is present in many lines and types of corn. This is the number in the mature kernel. No more foliage leaves are formed until the onset of germination. The primordia of the three "seminal," or secondary, roots that occur in the corn embryo, in addition to the radicle, are initiated 30 to 35 days after pollination. This completes the formation of organs of the embryo of corn.

THE BASIS on which characters, or traits, of plants or animals are inherited becomes clarified through an understanding of the chromosome mechanism during reproduction.

The formation of a zygote by the fusion of an egg and sperm nucleus is the mechanism by which germ plasm from two individuals combines to form the heritable complex of the offspring.

The divisions which precede the formation of the sex cells provide a basis for segregation of parental traits among the offspring in a precise and predictable manner. In fact, geneticists observed the precise manner of segregation long before they observed the cellular mechanism of heredity under the microscope.

SEED COLOR of corn gives an example of the mechanism of heredity. One should remember that the seed of corn has three types of tissue: The outer covering, or pericarp, which is the ovary wall of the mother plant; the embryo, which contains equal germ plasm from the sperm and egg; and the endosperm, in which the germ plasm consists of two-thirds from the female and one-third from the male parent.

A single gene locus in chromosome No. 1 determines whether the pericarp of the kernel will be pigmented.

Most commercial corn carries in each of the homologous No. 1 chromosomes a gene that provides colorless pericarp on the kernel so that the colors of the underlying structures, usually yellow or white, give the characteristic color to the ears.

In some types of corn, however, the gene located at this locus has been altered to the degree that it causes the pericarp to have red coloration.

Let us assume that the ovule occurred on a plant having the normal yellow corn (which carries a colorless pericarp) and that the pollen grain that landed on the silk of this kernel of corn was from a plant carrying the gene for red pigmentation of the pericarp. A plant grown from a resulting kernel carries on one of its chromosome No. 1 homologs a gene that gives colorless pericarp. On the other homolog it carries a gene that gives red pericarp. The kernels of this plant will be red. The reason is that when the no-color and color-producing genes are in the same plant, the color gene has a dominant effect and suppresses the other factor.

During meiosis of the new plant, following the pairing of the two No. 1 chromosomes and their subsequent migration to opposite poles, two of the resulting megaspores carry color genes, and two carry no-color genes.

When the three megaspores disintegrate, there is an equal chance that the surviving megaspore will carry the color- or no-color-producing gene. When we consider all the kernels on an ear of corn, we can expect that half of the egg cells will carry a gene for red pigmentation and the other half will carry the no-color gene.

As we pointed out, meiosis also precedes the formation of pollen in the tassel of the plant. Half of the pollen therefore bears a no-color gene. The other half carries the gene for red grains.

Let us assume that we self-pollinate this plant: We collect pollen of the tassel and apply it to the silk in a way to keep away the pollen from nearby plants. There is an even chance that the egg cells will be fertilized with a sperm nucleus that carries a color gene. There also is an even chance that they will be fertilized by a sperm with the no-color gene. Because half of the egg cells carry the no-color gene and half of the pollen grains also carry this gene, one-quarter of the new zygotes on an ear of corn will carry the no-color gene in each of the two No. 1 homologous chromosomes.

It follows that one-quarter of the corn kernels will give rise to plants bearing yellow or white ears of corn.

Similar reasoning establishes that one-quarter of the new zygotes will carry the color gene in both of the homologous chromosomes, and the resulting plants will have red kernels.

Finally, one-half of the new zygotes will be heterozygous—that is, they will be carrying the colorless factor on one chromosome and the color factor on the other. Because of dominance of the color gene over the colorless gene, this class of zygotes will give rise to plants with kernels that have colored pericarps.

If the pigments of underlying kernel parts were yellow in both parents, we would therefore expect three-quarters of the progeny of the self-pollinated ear of corn to give rise to red ears of corn and one-quarter of the progeny to bear yellow ears. Here we have the familiar 3:1 ratio that occurs with the segregation of gene pairs when one of the genes is completely dominant over the other. Such ratios, called Mendelian ratios, were discovered by Gregor J. Mendel in Austria about 100 years ago.

When a plant is heterozygous for two pairs of genes located on two different chromosome pairs, both of which may affect the same character or two different characters, their assortment during meiosis is completely independent and at random. When the plant is self-pollinated, the proportion of segregates of the various types in the resulting offspring can be predicted by simple mathematical calculations.

Many characters are determined by a large number of genes. The precise influence of any one gene cannot be measured. Studies on the inheritance of such characters are restricted to quantitative measurements that reflect the aggregate effect of a number of genes. Inheritance of characters controlled by many genes is known as quantitative inheritance.

Some genetic factors carried by the generative nuclei of pollen grains have an immediate effect on characteristics of the developing endosperm. It will be recalled the endosperm develops from a triple-fusion nucleus, two components of which are provided by the ovule parent and one component by the pollen parent. When the pollen parent carries a dominant endosperm character and the ovule parent carries the recessive counterparts, the effect of the pollen parent is observable in the developing seed. This immediate effect upon the endosperm is known as xenia.

If the ovule parent of the corn, for instance, carries the genetic factor resulting in white endosperm and the pollen parent carries the corresponding gene for yellow endosperm, the developing endosperm will have a light yellow appearance. If the pericarp is colorless so that the endosperm color can be seen, the end result will be that a yellow ear of corn will be borne on a genetically white plant.

Other factors that produce xenia have been found in corn. Several brown and blue colors exhibit xenia when crossed onto white corn. Similarly, the genetic factors that condition starchy field corn will exhibit xenia when crossed onto plants of sweet corn.

EARLIER we mentioned the linear arrangement of genes on the chromosomes. The genes conditioning two characters may be located on the same chromosomes. If the two genes are located close to each other on the chromosome, the two characters they control in most instances will be inherited together. This phenomenon is called linkage.

At times, however, the characters are not inherited together, particularly when the locations of the genes on the chromosome are distant. A disruption of linkage results when an exchange occurs between the chromosomes in the region separating the genes. It is called crossing over.

When the homologous members of a pair of chromosomes begin to separate during reduction division, we mentioned that separation occurs first in the centromere region and subsequently the chromosomes uncoil. During the uncoiling process, numerous breaks may occur in the strands of chromonemata of the chromosomes, and the broken ends of chromonemata of homologous chromosomes may rejoin.

When such breaks occur in the chromosome region between two genes, a crossover occurs.

If the two genes lie at a considerable distance from each other, the frequency of gametes with crossovers may be as great as those containing the parental linkage, and from genetic evidence it will be difficult to determine if the genes actually are located on the same chromosome. The crossover mechanism provides a means of assortment of genes, even though they occur on the same chromosome.

IN CONCLUSION: Genes, which constitute the germ plasm and determine the hereditary traits of plants, are highly stable. Thought to be composed of complex organic molecules, they have the capacity to divide and reproduce themselves in kind.

The chromosome numbers of plants also are highly stable. Throughout the evolutionary process in plants, mechanisms that assure constancy of chromosome number through innumerable cell divisions of a growing plant and during the processes of sexual reproduction have developed.

During the processes of sexual reproduction, means are present, however, for assortment, segregation, and recombination of genetic factors. Tremendous genetic variability thereby is provided within a species. Occasional mutations of genes also contribute to the variability.

Genetic variability is especially significant in the evolution of plants. Plants that carry the proper combinations of factors are provided with the maximum opportunity to survive. Genetic variability also provides the basis for the potential improvement that man can make within a species to adapt it for his specific uses.