Lipoidal Labellar Secretions in Maxillaria Ruiz & Pav. (Orchidaceae)

Strictly for the bees?

The many types of interaction between flowering plants and their pollinators continue to provide fruitful and fascinating topics for research. Thus, we take a further look at the work of Davies and colleagues (Cardiff University and Swansea Botanic Garden, pp. 439-446). Their paper continues their previous research on the pollination of orchids and the possibility of rewards for the pollinators. They have investigated labellar secretions in the genus Maxillaria, focusing on three species, M. acuminata, M. cerifera and M. notylioglossa. Although not yet verified from field observations, these are thought to be pollinated by small bees. The question then is whether or not these orchids offer a reward to those bees. A clue comes from the specific name cerifera, meaning �bearing wax� and the accuracy of that description is borne out by this work. The authors have first examined the floral labella by TEM, clearly demonstrating the presence of osmiophilic globules secreted by the papillae (which have many of the features of typical secretory cells). The presence of osmiophilic material in the secretions implies the presence of lipids, and this was confirmed by histochemical analysis: the secreted material was stained strongly by Sudan III, which is also indicative of lipids. The authors have exhibited admirable caution in not describing the secretion as waxy, the term �wax� having a specific biochemical connotation. They prefer the term lipoidal but they do note that the secretions have a waxy appearance. Finally they ask �What is in it for the bees?� Some authors have suggested that when waxy material is gathered by pollinating bees it is used in nest building. However, these authors raise the possibility that the secretions may be of nutritional value because, in addition to lipid, they also contain aromatic amino acids and protein. The fascinating story of plants and pollinators thus continues.