Leaf size in mosses is structurally constrained by cell dimensions and genome size.

Leaf anatomy is a key factor determining plant ecology. Cell size and number are related to leaf size in tracheophytes, but this has been little studied in bryophytes, which never reach large leaf sizes. We studied the main anatomical factors determining leaf size in mosses, and how this is related to their ecology. We measured cell and leaf dimensions in 287 moss species, as well as cell density, cell wall thickness and midrib length. These measurements were contrasted against different traits, highlighting growth form and genome size, and correlations among traits. Moss leaf size was positively correlated with cell size but negatively correlated with cell density. The longest moss leaves were always supported by midribs reaching or surpassing the leaf apex. Genome size was positively correlated with cell and, especially, leaf size. All these relationships were stronger in acrocarpous mosses. Leaf size in mosses is limited by the mechanical support provided by cell turgor and the midrib. Both mechanical support and effect of genome size were more important in acrocarpous mosses. Our findings suggest anatomy as a key linking factor between genome size and plant ecology.