The Thread of Life Through Seeds

MARTIN G. WEISS AND JOHN E. SASS.

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 chromonernata 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.