Hydrophobic cuticular ledges prevent water entering the air pores of liverwort thalli

Summary

Air pore geometry of 14 liverwort species was investigated using light, scanning and transmission electron microscopy to assess their ability to prevent water entering into the air pore and the intercellular space of the thallus. The air pores of 12 species [Marchantia polymorpha L., M. paleacea Bertol., M. nitida Lindenberg & Lehmann in Lehmann, Conocephalum conicum (L.) Wiggers, C. supradecompositum (S.O. Lindberg) Stephani, Grimaldia capensis Stephani, Targionia hypophylla L., Lunularia cruciata (L.) Dum., Reboulia hemisphaerica (L.) Raddi, Oxymitra paleacea Bischoff, Exormotheca bullata (Link) K. Müller (Tüb.), Plagiochasma elongatum Lindenberg & Gottsche] are equipped with hydrophobic ledges which constrict the air pore entrance after the fashion of an iris shutter. As a consequence only liquids having a contact angle of zero degree with the ledges are able to penetrate the air pore. The ledge of Marchantia paleacea was found to have a surface composed of methyl and methylene groups as indicated by the critical surface tension of less than 30.4 dyne cm^-1. Evidence is presented that the ledge is covered with a layer of cutin. It is shown that from the standpoint of protection against water entry these air pores must be considered perfect structures. Plagiochasma rupestre (Forster) Stephani and P. peruvianum Nees & Montagne are exceptional as they do not have ledges around their air pores. As a consequence liquids having a finite contact angle are able to enter through the pores into the thallus. Thus, they are imperfect and must be considered primitive.