SOMATIC MUTATIONS IN SUNFLOWERS 



T. D. A. COCKERELL 



University of Colorado, Boulder, Colo. 



WE USUALLY think that, after 

 fertihzation has taken place, 

 the characters of the resulting 

 individual are determined, ex- 

 cept so far as they may be modified by 

 environmental influences. There are, 

 however, many facts which show that, 

 while this is generally true, it is by no 

 means an invariable rule. Some of the 

 apparent exceptions are evidently mis- 

 leading, as when in a bird the loss or 

 disease of the sexual organs results in the 

 appearance of characters belonging to 

 the opposite sex. Such cases are to be 

 classed among those due to the environ- 

 ment, and have nothing to do with any 

 fundamental change in the nature of the 

 inherited qualities. It is quite other- 

 wise with bud-sports in plants, which 

 are capable, apparently, of modifying 

 the offspring of the seeds produced by 

 the affected branch. In all such cases 

 it appears that a change in the hereditary 

 constitution of the cells has occurred in 

 the soma or body, without having any 

 connection with the processes of sexual 

 reproduction. Such changes also ap- 

 pear to be the cause of gynandrcmorphs 

 in insects; those strange individuals 

 showing the characters of the male in 

 one part of the body, of the female in 

 another. Thus I possess a bee of a 

 species in which the clypeus or facial 

 plate is yellow in the male, black in the 

 female. In this specimen it is yellow 

 on one side, black on the other, the 

 dividing line running exactly down the 

 middle. To such changes, also, we may 

 probably attribute the disease known 

 as cancer, in which certain cells of the 

 body entirely abandon their normal 

 functions. Mouse cancer can be trans- 

 planted from mouse to mouse, just as 

 a branch of a tree arising from a bud 

 variation may be grafted on to another 

 tree. 



Professors Babcock and Lloyd, in 

 Journal of Heredity, February, 1917, 

 pp. 82-89, have discussed these matters 

 at some length, and object to the 



expression "somatic segregation" as 

 going beyond the present state of 

 our knowledge. The changes observed 

 could be due either to some modification 

 in a particular cell, due to causes not 

 ascertained; or they could result from 

 inequalities or errors in the process of 

 mitosis or cell-division. Are we at 

 liberty to assume that the latter 

 explanation is correct? It is doubtless 

 better to use a term such as "somatic 

 mutation," which does not commit us 

 to anything more than the fact of the 

 change. At the same time, it must 

 be significant that these phenomena 

 more commonly occur, as East has 

 pointed out, in plants known to be 

 heterozygous. Granting that mitotic 

 errors are extremely rare, if they occur 

 once in many millions of divisions all 

 the observed facts may be accounted 

 for. It is also significant that these 

 variations so often represent the appar- 

 ent loss of one of the known determiners 

 of the plant. This might, indeed, be due 

 to a mutation of the determiner itself, 

 but it is easier to believe that in a 

 heterozygous cell one of the pair of 

 diverse allelomorphs disappeared, and 

 that this disappearance was due to a 

 mitotic error. 



The red sunflower (varieties of Heli- 

 anthus annuus) is unusually favorable 

 material for the observation of somatic 

 mutation. I figure two rem.arkable 

 heads produced in Boulder by my wife 

 in 1917. In each case the dark rays 

 are deep chestnut red, more or less 

 tipped with yellow, while the pale rays 

 are yellow. In one it will be seen that 

 three rays are yellow, the rest red. 

 In the other it is about half and half. 

 The latter head is remarkable in several 

 ways. The yellow rays are weakly 

 suffused with red, showing that they 

 do not wholly lack the factor for 

 redness. Either they possess an inhib- 

 itor or they lack a factor for full color. 

 It will also be noted that on one side 

 the dividing line between the red and 



467 



