Development and Function of the Strumae on the Adaxial Sides of the Inner Petals of Oncodostigma hainanense

Scanning electron microscope (SEM) and light microscope (LM) were used to examine the development and function of the strumae on the adaxial sides of the inner petals of Oncodostigma hainanense. We found that no struma formed at stageⅠ and Ⅱ of bud development. However, obvious strumae were formed at stage Ⅲ, which protruded outwards from the plane of tissue. Strumae were fully developed at stage Ⅳ. The periodic acid-schiff (PAC) reaction indicated that polysaccharides increased with the formation of strumae and were mainly distributed in cells around the strumae at stage Ⅳ. We have observed some thrip larvae among mature strumae, but no secretive opening or secretion on the surface of the strumae was found. We speculated that the strumae on the adaxial sides of the inner petals provided flower visitors a nutritious and safe place for mating, oviposition, brooding and perhaps some food such as polysaccharides.