In vascular plants, sporangia are produced on the surfaces of leaves. In some plants, such as ferns, sporangia are located on leaves that also carry out photosynthesis. However, in conifers and some other plants, sporangia form on the surfaces of highly modified leaves and branches that form compact cones. In both cases, the arrangement of the sporangia-bearing leaves is the same as leaves that do not bear sporangia.

Flowers develop from floral meristems formed by the conversion of shoot meristems. This means that flowers can be produced at the tip of a shoot or at the base of leaves as the products of axillary buds. Floral meristems differ from shoot meristems in several ways. First, all of the cells within a floral meristem differentiate, and so floral meristems lose the capacity for continued growth. Second, although floral organs (sepals, petals, stamens, and carpels) develop from primordia and are thought to be modified leaves, their arrangement differs from the placement of leaves along the stem. Floral organs occur in whorls, with each whorl consisting of only a single organ type (see Fig. 30.12).

Flowering is triggered by florigen, a protein produced in leaves and transported through the phloem to apical meristems and axillary buds. Florigen triggers the transition to floral meristems by initiating the down-regulation of meristem identity genes and the up-regulation of genes that govern floral identity. Once the meristem is launched along the trajectory for flower development, the identity of each whorl is controlled by the expression of homeotic genes (Chapter 20).

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