What are the "sticky rings" on stems of Anulocaulis and related taxa (Nyctaginaceae) from arid regions?

Abstract

Anulocaulis, commonly known as “ringstem,” is a small, unusual genus restricted to the Chihuahuan, Sonoran, and Mojave deserts of North America. Here we combined light microscopy and histochemical tests to characterize for the first time the “sticky structures” (here called secretory rings) found on the stem internodes of Anulocaulis. The secretory rings were shown to be groups of epidermal cells, or unicellular glandular trichomes, which largely differ from their neighboring cells both in structure and histochemistry. The cells start to differentiate in early stages of stem development. They begin as regular epidermal cells, but later their anticlinal and external tangential walls start to enlarge. At maturity the cells become remarkably elongated, even balloon-like, with dense cytoplasmic content. Although the secretory rings have been reported as “mucilaginous structures” based on morphological observations, preliminary histochemical analyses showed that its exudate is complex, including a mixture of mucilage, proteins, and phenolic compounds. Future investigations are needed to compare the anatomy of the secretory rings within related genera of Nyctaginaceae and characterize the chemical components of their exudate more specifically to search for potential homologies and adaptive functions of these structures.