266 GENETICS IN RELATION TO AGRICULTURE 



The Nature and Causes of Factor Mutations. Our knowledge of 

 genetic factors is entirely of an inferential sort and it is probable that 

 these ultimate hereditary units are no more likely to be objectively 

 perceived than are the atoms of which all matter is generally believed 

 to be composed. But our present understanding of biochemistry and the 

 chromosome mechanism of heredity leaves no room for doubt concerning 

 the theoretical nature of these factors. Living protoplasm is generally 

 considered as composed of very complex organic compounds. The 

 phenomena of stereochemistry, especially the substitutional or cyclic 

 changes which occur within various compounds under proper con- 

 ditions, suggest that similar compensatory relations exist between 

 the substances composing the living cell. Yet cytological observations 

 indicate that the chromatin is the only permanent constituent of the 

 nucleus and that the chromosomes are unaffected by the regular physio- 

 logical processes of metabolism, growth and reaction to stimuli even 

 though they play a very definite role in all these activities. As was 

 explained in Chapter IV, the chromosomes are linear series of loci whereat 

 are located specific factors. According to the multiple allelomorph 

 hypothesis more than one factor may exist at a given locus. Since the 

 chromosomes appear to consist of the only permanent substance in the 

 nucleus, it is conceivable that at each locus there exists a unique chemical 

 system; yet it is not unreasonable to suppose that occasionally substi- 

 tutional changes similar to those known to take place in less complex 

 organic compounds may occur here. 



The contributions of Reichert on the specificity of proteins and 

 carbohydrates as a basis for the classification of animals and plants are 

 based on the fact that such substances as serum albumin, hemoglobin, 

 glycogen and starch exist in stereoisomeric forms. That is, "each kind 

 of substance may exist in a number of forms, all of which forms have the 

 same molecular formula and the same fundamental properties in common, 

 but each in accordance with variations in intramolecular configuration 

 has certain individualities which distinguish it from others. ... It has 

 been found that the number of possible forms of each substance is de- 

 pendent upon the possible number of variations of the arrangements of 

 the molecular components in the three dimensions of space, or, in other 

 words, of variations of molecular configuration, the possible number in 

 case of each substance being capable of mathematical determination. 

 Thus, we find that serum albumin may exist in as many as a thousand 

 million forms. Hemoglobin, the red coloring matter of vertebrate blood, 

 is a far more complex carbon compound than serum albumin, and theoret- 

 ically may exist in forms whose number is beyond human conception, 

 running into millions of millions. The same is true of starch." Having 

 in mind this complex molecular structure of protoplasmic constituents 



