DIVERSITY OF INTERNODE INDIVIDUALS 



O. F. Cook 

 Bureau of Plant Industry. U. S. Department of Agriculture 



DATA of special value in the 

 study of diversity may be drawn 

 from plant life or from such 

 lower animals as worms, millipeds, or 

 centijieds, with the body showing num- 

 erous repetitions of the same organ. 

 Dozens or hundreds of joints of a 

 metamerous plant or animal body may 

 follow each other as exact biological 

 equivalents, and such series may be 

 considered as the closest approach to 

 uniformity in the processes of repro- 

 duction. Since the metamers are prod- 

 ucts of segmentation or vegetative 

 growth, the problem of interpretation 

 is greatly simplified. Many physiologi- 

 cal and environmental complications 

 attend sexual reproduction, with end- 

 less possibilities of confusion regard- 

 ing the heredity of sexually produced 

 individuals. The individuahty of the 

 vegetative internodes is of a lower 

 order, but lends itself better to a deter- 

 mination of the question of diversity. 

 That internode members of the 

 same plant are formed under the same 

 conditions and are of the same hered- 

 ity, is hardly to be questioned. At 

 least, it will be admitted that the con- 

 ditions of internode development are 

 more alike in the same plant than can 

 be asserted for different plants. Of 

 course, the vegetative internodes may 

 be specialized in various ways and de- 

 grees, as are the internode components 

 of the flowers and fruits, so that num- 

 erous types of internodes may be dis- 

 tinguished among the internode mem- 

 bers of a plant, but hundreds or thou- 

 sands of examples of the same type 

 may be available for comparison in a 

 tree, shrub, or large plant. 



Biological Duplication 



Not only do successive internodes or 

 leaves of the same vegetative shoot rep- 



resent biologically the same thing, but 

 comparison of the two halves of the 

 same leaf or other symmetrical parts 

 of the same internode individual gives 

 even a sharper focus upon the question 

 whether biological duplication or mani- 

 folding of the same structure is a pro- 

 cess of mechanical exactness or may 

 follow many alternative courses, with a 

 resulting diversity of detail. 



Whether we think of a transmitted 

 "character" as a general "determiner" 

 of leaf-form for the entire plant, or as 

 restricted to a particular feature, or to 

 an individual leaf, exact determination 

 is hardly to be claimed when definite 

 difi^erences are shown by corresponding 

 parts of the same leaf. (See figures 

 19 to 22.) 



The meaning of such facts in rela- 

 tion to heredity depends, of course, on 

 the assumptions that are made regard- 

 ing the nature and number of the trans- 

 mitted determiners or "genes." In 

 proportion as fewer determiners are 

 assumed for the transmission of the 

 characters a wider range of diversity 

 must be admitted in the expression or 

 development of the characters, to ac- 

 count for differences among the inter- 

 node individuals of the same plant. 

 The course of internode development 

 is not simple and direct, but divides and 

 ramifies into the many alternative paths 

 that are shown in the formation of the 

 different types of internodes, as well as 

 in the diversity of form that appears 

 among the internode individuals of the 

 same type. Even in the same leaf, 

 diversity may be shown, when equiva- 

 lent parts are compared. 



The dift'erent types of internodes on 

 the same plant may be as dift'erent as 

 species, genera or families, or may be 

 considered as analogous to the sexes, 

 castes or other definitely diverse forms 



322 



