Premise of the study: The seedling stage is one of the most critical phases in the life history of plants; during this stage, plants must develop efficient conductive and storage systems before the end of the favorable season. Little is known about the origin of successive cambia in seedlings of tree species of Ipomoea or about how many cambia are produced in one growth season. We studied the anatomy of Ipomoea arborescens seedlings to defi ne when cambium is differentiated, to determine how many cambia differentiate in one year of growth, and to relate the development of successive cambia to physiological aspects of growth.
Methods: Seedlings from 5 to 425 d of age were harvested, and their morphology as well as CO(2) and water exchange, were evaluated at 5 and 60 d after germination. Six stages of development were established to study origin of cambia.
Key results: Cambium was differentiated 5 d after germination, at a time when seedlings had photosynthetic cotyledons with high specific area, assimilation rate, and stomatal conductance. Differentiation of the fi rst successive cambium occurred inparenchyma cells below the endodermis or starch sheath. Development of reverse cambium and intraxylary phloem cambiumdemonstrated that ontogenetic shifts may occur in different stem regions. In the 10-mo-old plants, all cambia reactivated, and earlywood wide vessels were differentiated.
Conclusions: The origin of successive cambia, the occurrence of more than one type of cambium, and parenchyma proliferation are features shared by I. arborescens with its climbing ancestors as a strategy for survival in the harsh environment of tropical deciduous forests.