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ELEMENTS OF HEREDITY AND PLANT-BREEDING

All plants are made up of thousands of cells. These cells differ from one another in size and shape, each having its own particular function. Many different types of cell are found in every part of each plant, whether it be the roots, branches, leaves, petals, sepals, stamens, stigma, seed-pod, or any other part. The different types of cell in the one part of the plant work in co-operation. Some form tissues that bring water and soil-derived food solution into the plant; some form tissues that impart rigidity; some work in the absorption of plant food from the air; some make up the colourful attrac­tive flower; some make up the stamens, others the stigma, and so comprise the main reproductive organs of the plant.

Each cell contains a small body called a nucleus. Each nucleus contains minute rod-shaped bodies known as "chromo­somes", so called because they stain readily with dyes for microscopic inspection. In reproduction, it is these chromo­somes that transmit hereditary traits, or Mendelian factors. The alteration of any one chromosome will alter some char­acteristic of the plant, such as habit of growth, colour of flower, shape of petal, or number of petals. The number of chromo­somes in the cells of different creatures varies. In any cell, of any type of rose, it is always some multiple of seven; it ranges from fourteen to forty-nine. Roses which have different num­bers of chromosomes in their cells do not cross-fertilize readily. The great advances in rose-breeding have been due to unusual crossings between two types or species-a crossing that may possibly never be repeated. The new hybrid has been used, in each instance, as a parent for many re-crossings. These hybrid roses and most other hybrid plants differ in one important respect from hybrid animals, such as the mule, the liger, the tigron and others, in that they retain fertility in at least some degree.

Each rose bloom, like most other flowers, is bisexual-it is both male and female. The sex organs are in the flower and the calyx. The male organs are the stamens. They bear the anthers containing the pollen, which consists of the male cells for fertilization. The female organs are the pistils or styles. They are stalk-like, more or less tubular prolongations from the seeds within the calyx or seed-pod to the centre of the flower. They are usually coherent into one column, the stigma. The seeds in a calyx vary greatly in size, shape, colour, and number, from one to about thirty. Stamens surround the stigma in the centre of the bloom.

Neither pollen nor pistils are fertile until maturity. When a mature pollen cell reaches a mature receptive female cell (egg, ovary, or seed) their two nuclei fuse and the chromo­somes from one cell add to those of the other, making a total that will persist in the body cells of the plant that will come from the seed now fertilized.

When a germ cell is formed the chromosomes of the body cells are halved in number. There is no division of individual chromosomes into halves. The chromosomes separate at ran­dom, and so each germ cell formed has a different assortment of chromosomes and hereditary factors with which to unite, in fertilization, with another germ cell. Of course the vast majority of germ cells are lost. Some encounter a germ cell with which they are incompatible, and no fertilization occurs. This accounts for many failures in rose-breeding.

Each chromosome carries a Mendelian factor, which is said to be either dominant or recessive. A dominant factor will assert itself more than a recessive factor in a new variety, and so a crossing seldom produces a rose with characteristics mid­way between those of its parents.

The raising of roses from species is relatively simple and un­complicated. Self-pollinated seed from them produces seed­lings true to type, for there is no complex ancestry. Crosses of these parents show definite characteristics of both parents. Mendel proved that the first generation of these crosses gives the dominant factor in prominence in three-quarters of the seedlings. When these are self-pollinated one-third of the progeny still shows both the dominant and recessive factors. The one-quarter of the original group of first-generation crosses that showed recessive factors, when now self-pollinated, all produce progeny of their same type.

This is diagrammatically represented in Mendel's Principles of Heredity by Bateson, to the third generation thus:

"D" represents the dominant factor and "r" the recessive factor. When two factors, dominant for the same characteristic, meet in a crossing the resulting progeny are pure for that char­acteristic but will vary greatly with the recessive factors present. The same applies to the meeting of factors recessive for the same characteristic. Recessive factors can be asserted in the course of further cross-fertilization. With each succeeding generation, whether from artificial or self-pollination, the proportions and relationships of the chromosomes in the progeny become in­finitely more complex.

Modern cultivated roses are the result of many crossings and recrossings. Dominant and recessive factors have become highly complex. Often there are latent qualities suppressed through the presence of not only a dominant antagonistic factor but of a totally different factor. These unobtrusive factors often become apparent and important in later generations.

No two creatures are ever exactly alike. In creatures of the one variety and apparently alike the differences are so small as to be indiscernible, but they exist nevertheless. Their chromosomes are always in slightly different combination and there are always the latent powers of the recessive factors.