Cook : Evolution of Compound Leaves 



85 



effects on the organism, some of which 

 are not easily discoverable." This is 

 another way of saying that the problem 

 of expression of characters should be 

 separated from the problem of trans- 

 mission. The "effects" may be changed 

 bv altering the expression-relations of 

 the characters. To designate such var- 

 iations, that show intermediate stages 

 and combinations of characters, the 

 word metaphanic has been suggested.^ 



Evolution Through Mataphanic 

 Variations 



Special evolutionary interest may be 

 claimed for metaphanic variations, since 

 the development of new somatic fea- 

 tures may go forward through such 

 changes in the expression of characters, 

 without the need of supposing that 

 definitely new characters have been 

 formed in the "mechanism of heredity," 

 which serves for the transmission of 

 characters. Since many evolutionary 

 changes could be explained by suppos- 

 ing that characters already existing 

 have been readjusted or recombined in 

 their expression, fewer fundamental 

 changes in heredity would be called for, 

 if metaphanic changes were recognized. 

 No doubt many metaphanic variations 

 are abnormal or degenerative, as when 

 a loss of specialization results from 

 parts becoming more alike, but some 

 of the variations may be progressive. 



An example of evolution that may be 

 ascribed to metaphanic changes is the 

 development of the jointed rachis of 

 the compound leaves, which may be 

 viewed as an intermediate expression 

 or partial reflection in the leaf of the 

 jointed, internodal structure of the 

 stem. In the course of their develop- 

 ment, the compound leaves of the wal- 

 nut family have become more stem- 

 like or branch-like, contrasting in this 

 respect with other families where the 

 tendencv is for branches to become 



more leaf-like. In such cases as Phyl- 

 lanthus, Zizyphus and Castilla the 

 branches have not only the general 

 form of compound leaves but also the 

 temporary and deciduous behavior of 

 leaves, through the development of a 

 specialized joint or abscission layer at 

 the base. 



Cotton, coffee, cacao and many other 

 plants are like Zizyphus in having two 

 or more distinct forms of branches. 

 Intermediate forms of branches may 

 occur in abnormal cotton plants, and 

 such branches may be completely sterile 

 like hybrids of diverse species. In less 

 abnormal cases flower-buds may be 

 formed at each point of the interme- 

 diate branches, but all the buds are 

 aborted and shed before the fruiting 

 stage is reached, as in the so-called 

 "bull-stalks" of Sea Island cotton. 

 Sterility, however, depends to some ex- 

 tent on the external conditions, since 

 a few bolls may be set late in the sea- 

 son on plants that have aborted all of 

 the earlier buds.'' 



Metaphanic variations show that the 

 problems of adaptation are not merely 

 external, in relation to the environment, 

 since there also are problems of in- 

 ternal adjustment, or relations of the 

 parts to each other. With the princi- 

 ple of metaphany recognized, a wider 

 view is possible of the relations of the 

 characters or parts of an organism to 

 each other, since parts or characters 

 of independent origin may be com- 

 bined or rearranged through metaph- 

 any. The metaphanic variations show 

 a general lability and interplay of the 

 characters in their expression relations, 

 which are significant alike for hered- 

 ity and evolution. 



^Metaphany may be defined briefly 

 as internal hybridism, resulting in the 

 formation of intermediate structures, 

 so that specialization is reduced in the 

 sense that metaphanic variations render 



^ Brachysm, A Hereditary Deformity of Cotton and Other Plants. Journal of Aqricul- 

 tural Research. III. 387. February 15, 1915. 



^ See, Causes of Shedding in Cotton. Jounial of Heredity, 12:119, May, 1921. Also, 

 Dimorphic Branches of Tropical Crop Plants, Bui. 198, Bureau of Plant Industry, U. S. 

 Department of Agriculture, 1911. 



