Enzymes 311 



Van Overbeek has also demonstrated another auxin relation- 

 ship in maize. A recessive gene, na, when homozygous, produces 

 dwarf plants. Studies of the auxin content of the stems show 

 that these dwarfs form auxin but apparently have an enzyme 

 which causes it to be oxidized. The oxidized auxin has little 

 or no growth-stimulating effect, and the unoxidized auxin con- 

 tent of nana plants is lower than in normal plants. This reduced 

 amount of active auxin prevents the cells of the internodes from 

 elongating to the same extent as cells in a normal plant, and the 

 net result is a plant with shorter internodes and therefore smaller 

 height. 



Enzymes 



It has long been known that the speed of certain chemical 

 reactions will be greatly accelerated or retarded if certain other 

 substances are present even in small amounts. Substances which 

 thus affect the rate of a chemical reaction are called catalysts. 

 Similar substances may be found in living organisms, and many 

 of them are very important in the metabolism of the plant or 

 animal. These organic catalysts, found in living organisms, are 

 known as enzymes. Hormones are very similar but act on the 

 organism in a different region from the one where they are 

 produced. 



That genes may act as enzymes or that they might produce 

 enzymes as intermediate products in the development of a char- 

 acter has long been postulated, but the actual cases in which an 

 enzyme has been identified are not yet numerous. One definite 

 enzyme has recently been demonstrated in white clover. Certain 

 chemical substances of the type known as glucosides can be con- 

 verted into hydrocyanic acid if an appropriate enzyme is present. 

 Chemical tests have shown that white clover plants possessing 

 a certain dominant gene have a cyanogenetic glucoside in their 

 tissues and those homozygous for the recessive allele lack this 

 glucoside. Another dominant gene will produce the enzyme 

 whereas homozygous recessives will lack the enzyme. By ap- 

 propriate crosses, plants can be obtained which (1) possess both 

 glucoside and enzyme, (2) possess the glucoside but not the en- 

 zyme, (3) possess the enzyme only, or (4) possess neither the 

 enzyme nor the glucoside. These plants are good illustrations 

 of the direct production of a specific enzyme by a certain gene. 



