2 BULLETIN 1489, V. S. DEPARTMENT OF AGRICULTURE 
With a better knowledge of heredity, different systems of corn 
improvement have been tried from time to time, and the possibilities 
of progress have been tested by careful experiments. It is the pri- 
mary purpose of this bulletin to discuss these systems of corn breeding 
and consider their possibilities and limitations in agricultural prac- 
tice. It is desirable to consider first, however, the underlying prin- 
ciples on which corn breeding is based. 
PRINCIPLES OF CORN BREEDING 
Any progress that may have been made in methods of corn breed- 
ing has resulted from a better knowledge of the underlying princi- 
ples of genetics and cytology, achieved through intensive research not 
only with corn but with various plant and animal forms. Some 
knowledge of these principles is essential to an understanding of 
corn breeding. This involves a consideration of reproduction and 
inheritance in corn. 1 
REPRODUCTION IN CORN 
The growth of corn plants from the seed is but the most obvious 
step in reproduction. In common with the seeds of nearly all higher 
plants, each kernel of corn is produced as a result of the union of 
sexual cells, and it is only after this union, or egg fertilization, that 
the different parts of the kernel develop. 
THE CELL AS THE PLANT UNIT 
The corn plant is built up of minute units, the cells. These differ 
in detail and function in different parts of the plant but are alike 
in general structure. Cell structure is shown in Plate 1, A and B, 
which are reproductions of microphotographs of tissue from the root 
tip of a corn seedling. Each living cell contains a specialized portion, 
the nucleus. If a bit of rapidly growing tissue from a corn plant is 
properly prepared and stained, a number of rodlike or threadlike 
bodies within the nuclei of some of the cells may be seen with the aid 
of the microscope (pi. 1, B). These are called chromosomes because 
of the fact that they stain a dark color with certain dyes, whereas 
other parts of the cell remain unstained. The chromosomes are of 
special interest because of their relation to inheritance. 
CELL DIVISION 
Growth of plant parts is due to an increase in the number and 
size of cells, the number being increased by the division of preexist- 
ing cells. Cell division is not a haphazard affair, but a beautifully 
systematic process, during which each chromosome splits longitudi- 
1 It has seemed undesirable in a general publication of this kind to attempt a detailed 
review of the literature of the subject. Accordingly, citations are given in the text only 
as direct reference is made to specific data, suggestions, or conclusions. In addition, 
references to publications in which more extensive lists of literature dealing with the same 
subject may be found are given at the end of various divisions. Mention also should be 
made of the many excellent texts and monographs now available on genetics, cytology, 
and plant breeding. These afford a far more comprehensive presentation of the under- 
lying principles of plant and animal improvement than is possible within the scope of 
this bulletin. Among them may be mentioned ! h 20, 28, J f l, 67, and 71. (Reference is made 
by italic serial numbersi to " Literature cited," page 59.) 
