Are the spurs more complex than other petal types in Epimedium? Evidence from development, micromorphology, and nectary structure

As important organs attracting pollinators, spurs are the tubular outgrowths of flowers with rich diversities in color, shape, and size, and can increase reproductive success. The spurred petals in Epimedium L. (Berberidaceae) have diverse shapes and lengths, and others such as flat, shallow pockets, or saccate structures can also be found. Different shapes of petals may mean different features and functions, but research on petal development and structure is lacking in this genus hindering further research. Here, the petal development, micromorphology, and nectary structures of 9 species in Epimedium are observed to clarify the petal diversity. The results indicate that petal development can be divided into 5 stages, a concavity occur at different positions and then deepens in varying degrees during stage 4, which leads to petal differentiation. Petal stereoscopic structures were influenced by both cell division and anisotropic expansion in early developmental stages, the latter contributed greatly to spur elongation. There are 7 types of cells observed in Epimedium, with only 1 type of secretory cell; tabular fusiform cells appear in all 9 species; conical cells are widely distributed on the petals with nectaries, while papillose conical cells are only observed in spurs. Nectaries are usually distributed at the tip of the petal stereoscopic structure, consisting of the secretory epidermis, 2 to 13 layers of secretory parenchyma cells, and vascular tissues. These results help us to understand the morphogenesis and development of Epimedium, and provide morphological evidence for future studies on the phylogeny and evolution within the genus.