8 DIMORPHIC BRANCHES IN TROPICAL CROP PLANTS. 



STRUCTURAL SIGNIFICANCE OF DIMORPHIC BRANCHES. 



In attempting to understand the dimorphism of branches it is 

 desirable to consider the nature of the structural units that compose 

 the bodies of the plants. For some of the purposes of scientific study 

 the individual cells or the tissues formed by the cells of one kind can 

 be considered as units of structure. But many forms of plant and 

 animal life also show structural units of a higher degree, such as the 

 many similar joints or segments that compose the bodies of the worms 

 and centipedes and the internodes of higher plants. Each joint is 

 highl}' complex in itself, with a complete system of tissues and 

 organs. The word " metamer " is used as a general term to apply to 

 these complex units of organic structure. In some of the lower forms 

 of animal life each metamer is capable of an independent existence, 

 just as in some plants each joint of the stem or the rootstock, if 

 planted as a cutting, will grow into a new individual. In a similar" 

 way each seedling represents a single metamer, able to produce 

 others. 



Two general groups of metamers may be recognized in plants — 

 those that build up the vegetative parts of the plant and those that 

 take part in the formation of the flowers and fruit. A vegetative 

 metamer consists of a joint or section of the stem, together with a 

 root or roots, and one or more leaves, as well as the hairs, scales, and 

 other smaller appendages that belong to the joint, the root, or the 

 leaf. 



The floral or reproductive metamers of plants are generally smaller 

 than the vegetative metamers. The part that corresponds to the joint 

 or section of the stem of a vegetative internode is extremely short, 

 while the part that corresponds to the leaf takes the form of a sepal, 

 stamen, or pistil. 



A plant as a whole represents a collective individual — a social 

 organization, as it were — of the different kinds of subordinate meta- 

 meric individuals, some devoted to vegetative purposes and some to 

 reproduction. Botanical wa-iters have often referred to the floral 

 organs as transformed leaves, but it is quite as reasonable to suppose 

 that the leaves represent floral or reproductive organs that have 

 assumed vegetative functions." 



The stamens and pistils of the primitive types of plants are more 

 nearly like those of the advanced types than are the vegetative meta- 

 mers, showing that evolution has tended more toward the specializa- 

 tion of the vegetative parts. Dimorphic branches represent a some- 

 what advanced stage of vegetative specialization. A plant with 



" Cook, O. F. Origin and EAolution of Angiosperms through Apospory. Pro- 

 ceedings, Washington Academy of Sciences, vol. 9, 1907, pp. 150-178 

 198 



