Asteraceae

• The most diverse and ecologically significant family of woody and semi-woody shrubs in creosote bush scrub is the sunflower family. Globally Asteraceae is second only to the Orchidaceae in number of species. Memers of the Asteraceae family are challenging to identify until one becomes familiar with flower structures. The essential feature defining the Asteraceae is the presence of composite flower heads that typically include numerous tiny flowers joined in what, at first, resembles a single large flower-as seen in a familiar sunflower. The typical composite flower head is composed of two types of flowers, although either one of these types may be absent in some genera.
• The outer whorl of flowers in a typical composite head has three petals fused to form what appears to be a single large petal, while the other two petals are absent. These fused petals of the ray flowers, or ligules, are the attractive structures for pollinators. Ray flowers are almost always female. The inner groups of flowers are called discoid and consist of five tiny petals fused into a tube with radial symmetry and only their tips separated. Disk flowers typically include both male and female parts. The flower heads, which are found in the majority of species with both disk and ray flowers, are called radiate heads. Discoid heads are composed of only disk flowers and, although they lack showy the showy ligules, can nevertheless be colorful and effective at attracting pollinators. There are a few genera of Asteraceae where disk flowers are entirely absent. In these all five petals fuse to form single ligule as a variant of typical ray flowers. In these cases, with dandelions being a familiar example, the ligulate flowers are all bisexual with both male and female organs. The presence of either radiate or discoid flower structures is a key trait in identifying members of this family.
• The individual flowers within the heads of Asteraceae lack typical sepals. Instead, the organ that would give rise to sepals have been modified to scales, hairs, or bristles called pappus. This pappus attaches to the single-seeded ovary and readily attaches to the fur of animals. The plumed white parachutes of dandelions, and many bristly stickers that attach to hikers' socks, are examples of a specialized pappus. The efficiency of these structures in dispersal makes members of the Asteraceae effective colonizers of open habitats. "California Desert Plants, Pihlip W Rundel, Robert J Gustaffson, Michael E Kaffmann."