From the Digest

Tube or Not Tube

By Joe Merkelbach (February 2010)

Continuing with the idea that cacti and other succulents have their converging similarities, as well as marked differences, the flowers certainly indicate that not all the forms are closely related.

Cactus flowers are always tubular in form, although they vary from deep to quite shallow and open. Most of the flowers are also quite showy, particularly many of the species that collectors value. They also vary in size from small to very large.

They always have large numbers of sepals, petals and stamens. The ovary is located deep within the tube, perhaps as a means of protection for development in harsh conditions.

The tube form develops as a sort of false stem with green bracts – actually leaf tissue – visible as scales on the exterior of developing buds. The vegetative tissue grows surrounding the bud and actually elongates faster than the flower parts.

The sepals and petals push out at the rim of the tube, but the numerous stamens and the pistil and ovary are located down inside the surrounding vegetative tissue. Generally, these false stem parts form a protective outer covering for the seeds inside. Big and flashy are good descriptors for cactus blooms.

Euphorbias have very different flowers. They are small to almost microscopic and located in a cup-shaped structure called a cyathium. The cyathia lack petals but contain fluid-secreting glands that attract pollinators as well as the very small male stamens and femal pistil. In some species, the pistil extends out of the cup on a stalk called a pedicel.

Some euphorbs, such as E. obese, are dioecious, with male and female flowers on separate plants. Tiny and subtle are good descriptors for euphorbia flowers.

Crassulas have blossoms grouped on a branching stalk with a symmetrical form of five sepals and five petals. They have a form that most people would call a “regular” flower look.

These extreme variations in flower form indicate that succulent plants are not at all closely related. Rather, they have developed their common, thickened water-retaining shapes due to the environmental pressures of harsh, arid habitats all over the world.

University of Texas School of Biological Sciences –
Wayne’s Word –