In tropical forests, vines weave their way upward through the branches of trees. Many of these vines have long, thin stems with widely spaced leaves (Fig 31.1a). In contrast, a barrel cactus, living in the desert, has thick stems and leaves modified to form sharp spines (Fig. 31.1b). Although they look nothing alike, these two plants are constructed in the same way. Plant shoots are modular, meaning that they are formed of repeating units. Each unit consists of a node, the point where one or more leaves are attached, and an internode, the segment between two nodes (Fig. 31.1c). In vines the internodes are long and the leaves large, whereas in cacti the internodes are short and the leaves thin and sharp. The modular structure of plants helps us understand how the capacity for continued growth gives rise to the tremendous variation in plant form that we see around us.