Male sterility is inherited as a recessive gene. Two types of cytoplasm are involved. When the gene for male sterility is present in a plant with normal cytoplasm, the plant produces normal pollen. The male-sterile gene functions only in the presence of a sterile type of cytoplasm, and viable pollen is not formed. Hybrid onion seed can be produced by planting a male-sterile variety with a normal variety that furnishes pollen.

The male-sterile gene has now been incorporated into many onion varieties, and hybrid onions are widely grown. The male-sterile bulbs are planted in four rows with a row of bulbs of the pollen parent on each side. Bees and flies carry pollen to the male-sterile plants, and all their seed is of hybrid origin.

The time of blooming varies with different onions. The pollen parent bulbs are planted a week or two earlier, depending on their flowering date. This insures that pollen will be available when the male-sterile plants come into flower.

The same general type of cytoplasmic male sterility as occurs in onions has been found in sugarbeets, carrots, corn, millet, orchardgrass, pepper, petunia, sorghum, tobacco, and wheat. Hybrid petunias are produced with male-sterile lines. Research has begun with beets, carrots, and other crops.

Plant breeders are aware of the potential possibilities of this method of producing hybrid seed. Undoubtedly male sterility will be discovered in other crops.

Polyploid plants are ones that have multiples of their basic chromosome number. Plants with twice the basic number are called diploids. Those with three sets of chromosomes are triploids. Those with four sets are tetraploids.

There are known instances of spontaneous doubling of the chromosome numbers of a primrose and of the poinsettia.

Some of our finest ornamental plants and many of our fruits are polyploids. Until about 1910, practically all varieties of garden iris were diploids. Larger flowered seedlings began to appear about this time in the plots of iris breeders. These large-flowered seedlings were named, and in 1943 the chromosome number of 109 of these new varieties was determined; 108 were tetraploid. One was a triploid.

The new varieties of poinsettia originated as bud sports and were propagated because of their superior characters. These new varieties have been found to be tetraploids.

The discovery in 1937 that colchicine would double the chromosome number of plants gave plant breeders the opportunity of exploring the possible usefulness of tetraploidy in improving crop plants.

Research workers found that the immediate results were not promising. Tetraploids of seed-propagated crops are highly sterile, especially those of normally self-pollinated crops. The tetraploids of many plants were inferior to the diploids, and some of the early enthusiasm regarding induced tetraploidy subsided. It was not generally recognized that induced tetraploids should be considered as raw material for continued breeding and selection.

MANY ORNAMENTAL plants are propagated from cuttings (asexual propagation), and fertility is not important in them. In fact, practically all asexually propagated ornamentals do not come true from seed because of their heterozygosity.

Induced tetraploids of such plants have shown some promising results. Several species of Lilium have been made tetraploid and are coming into use in gardens. A tetraploid forsythia produced by colchicine treatment at the Arnold Arboretum, Boston, Mass., bears larger flowers of a deeper golden color.

Tetraploid carnations from colchicine have sturdier stems and larger flowers. A few tetraploid carnations are on the market.

Induced tetraploids usually flower later than their diploids and produce fewer flowers per plant.

This was the situation in lilies, but after 15 years of continued breeding and selection, early blooming and floriferous tetraploid lily seedlings were developed. There has been a rearrangement of the genes in these lilies, and late blooming and lower flower production have been eliminated.

colchicine-induced tetraploids may be useful in breeding when one parent is diploid and the other is tetraploid.

In cranberry, there are three species: two are diploid and one is tetraploid. All attempts to cross the tetraploid with the diploids failed. The chromosomes of the diploid varieties were doubled, and some of these induced tetraploids were crossed with the tetraploids. These crosses are leading to new and improved varieties of cranberries.

Hybrids between species are usually sterile. The chromosomes of the two species, although able to form a new plant, cannot form functional pollen and egg cells. Doubling the chromosome number of such hybrids usually produces fertile plants that may lead to developing new varieties. Such sterile hybrids have spontaneously produced branches with fertile flowers. The branches have the double number of chromosomes.

Polyploid sugarbeets have been produced in Sweden and Japan. The tetraploids with 36 chromosomes were inferior to the diploids with 18 chromosomes, but when they were crossed with diploids, they produced useful triploids with 27 chromosomes. Swedish plant breeders have reported that after a long period of breeding and selection, useful strains of tetraploid sugarbeets are being obtained.

A triploid watermelon from Japan is available. It was developed by crossing colchicine-induced tetraploids with diploids. The cross is successful only with the tetraploid as seed parent. The triploid watermelons are seedless. They have a thin rind and high content of sugar.

THE PLANT breeder now has many techniques and tools to work with. The accomplishments of the past half century are many, but will probably be far exceeded as newer and more improved methods are devised.

S. L. EMSWELLER entered the Department of Agriculture in 1935. He is leader of Ornamentals Investigations. He came from the University of California, where he received his doctor's degree in 1932.