[Chap. XXXIX MUTATIONS 485 



ously crested leaflets. These plants also exemplify the fact that mutations 

 occur in vegetative organs, for these mutations occur when the ferns 

 propagate vegetatively from rhizomes and runners. 



Although the kinds of mutations are apparently independent of 

 changes in the environment, the rate at which they occur is influenced 

 by the environment. Very little is known about the comparative increase 

 in frequency of the several kinds of mutations. If the rate of certain 

 mutations is consistently increased more than that of others in a given 

 environment, one might expect that certain variations would accumulate 

 more abundantly than others in certain habitats, provided, of course, 

 that they are not eliminated more rapidly. 



Experimental means of speeding up mutations by decreasing or in- 

 creasing the temperature, by treating the plant with chemicals, or with 

 ultra-violet lisht, X-ravs, and radium emanations have enabled investi- 

 gators to obtain data about mutations much more readily. The idea that 

 mutations under natural conditions may depend upon cosmic rays has 

 already been discredited. It is interesting to note that plants from old 

 seeds, like those from seeds treated with X-rays, have more mutations 

 than plants that grow from freshlv harvested seeds. Apparently X-rays 

 speed up deleterious changes in seeds similar to those that occur natu- 

 rally with age. The effects of X-rays are the result of chromosomal aber- 

 rations and of the destruction of hereditary units of matter. 



Some genes are much more stable than others. When the colors of 

 variegated leaves, flowers, and grain coats are the visible effects of local 

 mutation, the various sizes of the pigmented areas indicate the amount 

 of tissue that developed from the cells in which the mutation occurred. 

 The occurrence, shape, and location of the white and pigmented areas in 

 the leaves of coleus, however, are seldom due to local mutations in the 

 leaves. These patterns are as definitely heritable through genes in the 

 gametes as are the pigments. There are pure lines of coleus with respect 

 to these patterns; and when they are crossed with each other, the pat- 

 terns recur in the usual Mendelian ratios in the progeny of the hybrids. 



The idea that only the best or "fittest" mutations survive has been 

 abandoned bv most students of plant physiology and heredity. So, also, 

 have the notions that a heritable variation must have some adaptive 

 value to survive, that the variation occurred to meet some need, and 

 that the variation was selected by nature. Numerous heritable variations 

 that neither interfere with nor contribute to the life of the plant occur 

 and survive, and their mode of origin is precisely the same as those 



