They have absent, small, or transient leaves. They have flowers with ovaries that lie below the sepals and petals, often deeply sunken into a fleshy receptacle (the part of the stem from which the flower parts grow). All cacti have areoles—highly specialized short shoots with extremely short internodes that produce spines, normal shoots, and flowers. The remaining cacti fall into only two genera, "Pereskia" and "Maihuenia", and are rather different, which means any description of cacti as a whole must frequently make exceptions for them. "Pereskia" species superficially resemble other tropical forest trees. When mature, they have woody stems that may be covered with bark and long-lasting leaves that provide the main means of photosynthesis. Their flowers may have superior ovaries (i.e., above the points of attachment of the sepals and petals), and areoles that produce further leaves. The two species of "Maihuenia" have small, globe-shaped bodies with prominent leaves at the top. Cacti show a wide variety of growth habits, which are difficult to divide into clear, simple categories. They can be tree-like (arborescent), meaning they typically have a single more-or-less woody trunk topped by several to many branches. In the genus "Pereskia", the branches are covered with leaves, so the species of this genus may not be recognized as cacti. In most other cacti, the branches are more typically cactus-like, bare of leaves and bark, and covered with spines, as in "Pachycereus pringlei" or the larger opuntias. Some cacti may become tree-sized but without branches, such as larger specimens of "Echinocactus platyacanthus". Cacti may also be described as shrubby, with several stems coming from the ground or from branches very low down, such as in "Stenocereus thurberi". Smaller cacti may be described as columnar. They consist of erect, cylinder-shaped stems, which may or may not branch, without a very clear division into trunk and branches. The boundary between columnar forms and tree-like or shrubby forms is difficult to define. Smaller and younger specimens of "Cephalocereus senilis", for example, are columnar, whereas older and larger specimens may become tree-like.