SIMILARITY TO ALTERNATING GENERATIONS. 11 



regularly appear among vegetative internodes of the same plant. 

 Dimorphic branches and similar specializations show that change of 

 characters in vegetative internodes is a definite phenomenon in the 

 development of plants, like changes that take place during the de- 

 velopment of many animals. Much evolutionary importance has 

 been attached by zoologists to the recapitulation of ancestral char- 

 acters in embryos, as well as to metamorphosis and alternation of 

 generations. All of these phenomena find their parallels among 

 plants, though botanists have given them relatively little attention. 



The evolutionary development of the various degrees of specializa- 

 tion of the branches of such a plant as the cotton becomes more com- 

 prehensible if we compare it with the stages through which a simple 

 herb would naturally pass in attaining the stature and habit of a 

 branching shrub or tree. Manv small herbs bear single terminal 

 flowers, but in plants that have increased in size and complexity ter- 

 minal flowers are replaced by axillary flowers or flower clusters, and 

 these tend in turn to grow out into branches, able to subdivide still 

 further and bear larger and larger numbers of flowers. 



In the cotton plant the primary branches have now become as 

 sterile as the main stem, and the extra-axillary branches that nor- 

 mally bear the fruit also have the power of changing over into sterile 

 limbs, the production of fruit being deferred to a later generation 

 of branches to enable the plant to construct a larger vegetative 

 framework. 



The main stem and the one or more series of vegetative branches 

 which intervene between the germination of the seed and the forma- 

 tion of another flower correspond to several generations of the vege- 

 tative parts of a simple herb and might also be compared to the vege- 

 tative generations of the plant lice and other lower animals that are 

 able to propagate for several generations by simple vegetative sub- 

 division, instead of requiring sexual reproduction for each genera- 

 tion of new individuals, as among the higher animals. The relations 

 between the sterile and the fertile branches of cotton and of other 

 plants that have dimorphic branches afford a rather close parallel 

 to the original examples of the phenomenon of alternation of gen- 

 erations, though they are not comparable to the changes that occur 

 in the life histories of the liverworts, mosses, and ferns which botan- 

 ical text-books commonly describe as alternation of generations." 



A shrub or tree may be thought of as a colony or complex of man}^ 

 individual branches each corresponding to a separate plant in a 

 species of smaller shrubs or herbs. Dimorj^hism of branches means 

 that there are two kinds of these branch individuals that follow each 



a Cook, O. F.. and Swingle, W. T. Evolution of Celhilar Structures. Bulletin 

 81, Bureau of Plant Industry, U. S. Dept. of Agriculture, 1905. 

 198 



