An interlamellar lipophilic layer regulates hygroscopic movements in moss peristomes

The moss peristome regulates spore release via the hygroscopic movements of the articulated teeth over the capsule mouth. The presence of hydrophobic material inside the teeth has been sporadically reported, with different names and compositions proposed, and it was thought to cause hygroscopic movements. Here we investigated the origin, characteristics and function of this deposition, now termed the interlamellar lipophilic layer (ILL), with the aim of understanding it and its contribution to peristome hygroscopic movements.

The ontogeny of the ILL in Dicranum scoparium was elaborated using histology and transmission electron microscopy. Histological, structural and physiological analyses of peristomes were conducted in 14 species of Bryopsida.

The ILL is formed simultaneously with the deposition of secondary thickenings in the developing peristome. In mature peristomes, the ILL resembles the cuticular layer of plant cuticles. The ILL is present in peristomes with single and double rings of teeth and its absence in three species was confirmed. The responses of the teeth under dry and humid conditions combined with the presence or absence of the ILL led us to reconsider its influence on the hygroscopic movements.

The presence of a polyester layer in moss peristome teeth is confirmed. We named it the interlamellar lipophilic layer (ILL). The absence of the ILL was revealed in three of the 14 species investigated. We confirm that hygroscopic movements occur when the ILL is present or absent. When the ILL is present the peristome teeth remain erect, whereas when it is absent the teeth are reflexed. These findings led us to hypothesize that the ILL influences the amplitude of movements rather than causing them, playing a regulatory role in the gradual dispersal of spores. The species that lacked the ILL were xerophytic, which may represent a novel spore dispersal strategy.